CNC Fiber LASER Assist Gas

Gas Purity

The purity of the assist gas is extremely important – impurities in the assist gas can create extensive damage to the cutting head, resulting in downtime and expensive repairs. Cutting head failures are the leading cause of machine downtime. Impurities in the assist gas (such as oil, solder flux residue, Teflon tape, and other impurities from the gas source, or the installation of the gas lines, end up on the cover glass (cover slide) in the cutting head. Here, they create hot spots on the cover glass and initially cause poor cut quality – and quickly lead to additional expensive damage if the cover glass breaks. For this reason, the purity required for laser assist gas should be higher than standard compressed gas cylinders.

Gas Pressure and Flow

Gas pressure and flow are important when sizing the system for delivery to the machine. Components must be sized to handle the flow rates and pressures required for the cutting process. It is highly recommended to size the delivery components to eliminate downstream pressure loss. Furthermore, components such as regulators must have large orifices to eliminate flow restrictions while cutting.

Gas Volume

Understanding gas volume is important to minimize downtime associated with replacing gas cylinders. Laser cutting uses a large volume of gas, so it is important to size the gas system with the optimum size to minimize disruption.

Investment Cost vs. Operating Expense

There are tradeoffs between the different styles of gas delivery systems. Each has a different set of investment and operating cost. It is in your best interest to understand the options, and choose a system that makes the most sense for your operation.

Application

It is important to understand which gases you will choose to use, and the options associated with each – all based on your application (the materials you wish to run on your machine)

For cutting carbon steel:

• The best quality is achieved with Nitrogen cutting
• In thinner materials, Nitrogen and Air can cut significantly faster than Oxygen
• The best combination of operating cost and speed is achieved with air cutting
• Air cutting can cut almost to the same maximum thickness as Nitrogen
• Oxygen cutting is the only option for thicker carbon steel
• Oxygen cutting has the lowest investment cost

Chart 1 – Process Capability – Carbon Steel

For cutting stainless steel and aluminum:

• The best quality is achieved with Nitrogen cutting
• The best combination of operating cost and speed is achieved with air cutting
• Air cutting is limited in thickness for stainless steel and aluminum
• Oxygen cannot be used for stainless steel and aluminum

Oxygen Systems

Oxygen systems are delivered to the cutting head at relatively low pressure (around 15 psi) and therefore the overall gas consumption (and therefore operating cost) is low. Flows range from 40 to 500 cubic feet/hour depending on the size of the nozzle. For example, a 1.0 mm diameter nozzle at 15 psi is used for cutting carbon steel up to ¼” thick at low power, and requires 40 scfh. Heavier pieces of carbon steel required “dual nozzles” (a secondary cooling ring of Oxygen). A 3.5 mm diameter dual nozzle is used in applications such as ½” thick carbon steel, and requires close to 500 scfh.

Oxygen can be delivered in compressed gas cylinders, liquid duers, or in bulk to an external tank. The choice can be made based on the expected volume of gas used, as well as the cost of the gas. Some important items to note here:

1. The purity of liquid oxygen is generally good enough for use as a cutting assist gas. Compressed Oxygen requires the use of special purities – many gas manufacturers now have special “laser quality” compressed Oxygen that achieves the required purity.
2. Liquid gases continue to “boil off” even when not being used. For this reason, the use of a liquid Oxygen dewar as a “backup” may not be a wise investment as much of the Oxygen may have been lost before you ever use it. Compressed gases maintain their volume over extended periods of time.
3. A single cylinder of compressed Oxygen contains approximately 330 cubic feet of Oxygen. For small diameter nozzles, this would be equivalent to about 8 hours of cutting. For thicker steel, it would drop to around 40 minutes of cutting.
4. Pressure control of Oxygen is much more important than with Nitrogen and Air cutting. Even slight variations in Oxygen pressure will affect cut quality.

The cost of cutting with Oxygen will vary depending upon the application. When using a small diameter nozzle (1.0 mm) and 15 psi, the usage is 40 scfh. If the cost of compressed gas is $2.00/100 cubic ft, the operating cost will be (40 / 100) x $2.00 = $0.80/hr. Similarly, if using a 3.5 mm dual nozzle at the same pressure, the operating cost will be (500 / 100) x $2.00 = $10.00/hr.

Nitrogen Systems

Nitrogen systems are delivered to the cutting head at much higher pressure (around 200 psi) than with Oxygen. Since Nitrogen does not offer any exothermic benefit for heating the material, its use is strictly for ejecting the molten material. As a result, the process is much more stable than with Oxygen, and extremely high cutting speeds can be achieved. However, for the same reason, the maximum cutting thickness is limited based on the power of the laser.

Using higher pressure results in significantly higher consumption. Flows range from approximately 650 scfh for a 1.5mm single nozzle to 2650 scfh for a 3.0mm single nozzle. In special cases (e.g. high power cutting of thick stainless steel) even larger nozzles may be used with significantly higher flows. Since Nitrogen consumption is so high, it is normally not practical to use compressed gas since cylinders would need replacement too frequently.

1. The use of portable liquid Nitrogen dewars are possible. These will require an in-line evaporator to turn the liquid into gas at a high rate to satisfy the high flow rates required by the process.
2. More often, customers use bulk liquid Nitrogen storage tanks outside their facility. These can be refilled easily and generally result in lower costs than portable dewars. The cost of bulk liquid nitrogen can be as much as 75% less than portable dewars, but there may be additional costs for a bulk tank rental, and potentially higher boil off. Furthermore, installation of the bulk tank, concrete pad, and distribution piping are required (one time cost).
3. In cases of high Nitrogen consumption, it is common for customers to purchase Nitrogen generators. These systems remove the Nitrogen from the ambient air, filter it, compress it, and deliver it to the machine. The purchase cost of Nitrogen generators are expensive, must be sized to handle the maximum anticipated flow rates, and its filtration system must be able to achieve the required purity level for laser cutting (99.99%). The operating cost of generating Nitrogen is significantly lower than purchasing liquid Nitrogen.

The cost of using liquid Nitrogen is dependent on the nozzle size. For example, in extremely light gauge carbon steel (20 ga / 1.0 mm thickness), a 1.5mm nozzle is recommended and the flow rate is about 650 scfh. If the cost of Nitrogen is $1.25/100 cubic ft, then the operating cost will be (664 / 100) x $1.25 = $8.30/hr. As material thickness increases, the required nozzle size increases, and therefore consumption also increases. The following table illustrates this effect (assuming $1.25/100 cubic ft):

Note: The price of Nitrogen gas varies greatly by region and fluctuates over time. Additional costs such as monthly bulk tank rental, delivery charges, and boil off are not included in the cost. These can add an additional $3.00 to $10.00 per hour when taken into consideration.

For multi shift operations, and thicker materials, the overall cost of Nitrogen usage may justify the purchase of a Nitrogen generator. For example, a 2 shift operation running mostly 10 ga through ¼” steel at 75% duty cycle will cost over $100,000 annually in Nitrogen usage alone. The purchase cost of a Nitrogen Generator system varies greatly based on required flow rates, pressure and purity. One can expect to pay between $100,000 and $300,000 for such a system.

Compressed Air Systems

Compressed air systems normally deliver the air to the cutting head at the same high pressure and flow as Nitrogen, and result in approximately the same cutting speeds. Cut quality is not quite as good as with Nitrogen, as the edge of the steel may have a grayish appearance and need additional edge cleaning prior to powder painting. However, compressed air systems are significantly less expensive than Nitrogen generation systems, and the net result is extremely low operating cost.

1. A dedicated compressor is required for laser cutting. The use of a standard industrial compressed air system (“shop air”) will not work and result in damage to the cutting head.
2. The compressed air system requires high purity. This is achieved by having a series of filters in line to achieve the required purity. The filtering system must be maintained or else the cutting head will be damaged.
3. Many laser cutting equipment manufacturers (such as Piranha) offer dedicated compressed air systems specifically designed for use with laser cutting equipment. These systems, while still costly, are significantly lower investment than Nitrogen generators. Typically, an air compressor system with dryer and filter will cost between $40,000 and $80,000
4. The compressed air system must be sized to achieve the maximum pressure and flow rate for the cutting application.

With compressed air systems, the hourly operating cost (for gas) approaches zero – the only ongoing operating cost is for the electrical power required, and for maintenance (e.g. filter replacement).

Nitrogen LASER Cutting

• High Pressure Bottles: Nitrogen is available in high pressure bottles. This is the most expensive and requires multiple bottles with a manifold to achieve the required gas flow rate.
• Liquid Nitrogen: Liquid nitrogen is available and the cost per cubic foot is dramatically reduced compared to high pressure bottles. The gas in the liquid packs will warm up and be released to the atmosphere when not in use and there is a loss of gas. Bulk systems place the nitrogen outside the facility and deliver it to the LASER via dedicated piping lines. The gas supplier usually leases the tank and support systems, filling the tank on demand or on a schedule. This is commonly used in multiple machine installations. Upfront costs for the facility and the lease should be evaluated.
• Nitrogen Generation: Nitrogen can be generated on-site using special nitrogen generation systems. The investment for a generation system is high, but the ongoing cost to produce the gas is very low, primarily the cost of electricity and system maintenance. One system will support one LASER.

Oxygen LASER Cutting

• Oxygen is used at much lower flow rates than nitrogen and can be supplied economically by high pressure bottles or liquid packs.

Air Assist LASER Cutting

• Air assist gas must be very dry and pure. Regular shop air is not used. A special compressor, dryer, and filtration system is required. Air cutting is practical when cutting thin carbon steel. For occasional use, high pressure bottles can be used.

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