Assist-Gas Technology Fuels Laser-Cutting Improvements

Page 1 Reprinted from the GASES & WELDING DISTRIBUTOR May/June 2004 issue. Assist-Gas Technology Fuels Laser-Cutting Improvements David Smith & Ross Petersen, Chart Industries, Inc. As power densities increase, motion controls advance, and materialhandling improves, distributors and end users alike can expect a better, cleaner, more efficient laser-cutting process. A s laser-processing systems continue to evolve, distributors and end-users can expect equipment and process modifications such as increased power densities, motion-control advancements, and material handling improvements. These changes will greatly influence process parameters and gassupply needs. For instance, new technologies have impacted the laser-cutting process. Four to 10kW lasers have been available for some time, but with poor beam quality, optical limitations, and expensive equipment and operation costs. Recently though, 4 to 6-kW, high-frequency, excited (HF) machines have made cutting a viable and cost-effective option. One HF laser advancement is a design that ensures the anode and cathodes are not in contact with the laser gases. This produces a consistently clean lasing process. As a result, HF machines have good beam quality with high-power density, quality pulse control, and an economical cost per watt. The higher power of HF lasers lets end users effectively cut carbon, coated, and stainless steels and aluminum with high-pressure nitrogen. Nitrogen in the cutting is an important

Page 2 Reprinted from the GASES & WELDING DISTRIBUTOR May/June 2004 issue. change in laser and assist-gas technology. Historically, oxygen was the assist gas for carbon-steel cutting. An exothermic reaction between the liquefied iron and oxygen provides a substantial portion of the heat required to cut steel. But, oxygen assist gas oxidizes the cut surface, and the heat generated can Distributor s distort the workpiece matelaser Checklist rial and remove surface coatings. Nitrogen miniselecting The Right Assist Gas mizes many of these cutsupply Source ting challenges because it What assist gas or gases is/are makes a cooler process. required? Nitrogen also increases What is the peak-operating processing speed and pressure required at the laser? produces oxide-free cut What is the peak flowrate? surfaces with little or no What is the cutting nozzle diameter? removal of surface materials. What pressure drop occurs Processing speeds with between gas source and laser(s) at nitrogen are directly related the peak flowrate? to laser-power density, What is the expected monthly material thickness, and asgas consumption? sist-gas pressure and flow. How much gas should remain The gas lets end-users in the tank at the time of refilling as process thin-gauge carbon safety stock? steels at faster travel speeds Can the supply of assist gas be with better edge quality, interrupted to fill the gas source? little or no surface deterio How often do you expect or want to deliver to the customer s site? ration, and less distortion. What are your expectations for This holds true for steel growing in gas consumption or thicknesses up to 0.15 in. pressure increases? When cutting thicker steels, distributors and end-users will find the heat from the oxygen/ liquid iron combination produces faster cutting speeds. Assist gas selection is important in assuring speed and quality. As such, it may be necessary to store and use multiple assist gases to optimize performance across a wide range of materials. In addition to these changes, the laser industry is also improving laser-system speeds thanks to motion-control technology. In fact, some high-power density machines now exceed 100 ft/min. In years past, a ball screw or rack- and-pinion system handled motion control, but maintaining high tolerances and repeatability at high process speeds became difficult. Incorporating linear motors solved this problem. They provide tight system control, repeatability, and high speed. Simple economics drove the third progression in laser systems automated material handling. Such systems employ dual pallets, ball transfer load stations, semiautomatic loaders, and fully automated loaders/unloaders. For instance, a multipallet automated load/unload cell operates in a lights out operation with little to no human intervention. Combining these types of automated material handling with more powerful lasers ensures 75%-plus duty cycles, high beam-on time, and economic viability of laser systems. The assist-gas requirements for advanced laser systems commonly exceed the parameters of traditional gas supply system. For example, the required assist-gas pressures can exceed 400 PSI (26.5 bar). Flows greater than 5,000 SCFH (131 Nm3) are required for certain material thicknesses. Figure 1 shows an estimate of cutting speed material thickness, gas pressure for cutting with nitrogen as the assist gas. The travel speed decreases as material thickness increases; however, assist-gas pressure increases significantly. The pressure and the cutting nozzle diameter determine the gas flowrate, and the nozzle diameter and assistgas flowrate increase with material thickness. Thus, the greatest flows and usually the higher pressures correspond to the greatest thickness processed. For the gas distributor and customer, gas volume consumed in a month is a third critical factor in selecting a gas supply system. With average gas consumption, the dis-

Page 3 tributor, along with the customer, must consider peak consumption and pressure, distribution schedules and costs, equipment costs, future growth expectations, and safety stocks. Selection of a suitable gas supply system requires knowledge of the maximum assist-gas pressure, the peak flowrate, and monthly volume of gas consumed, which can be determined by answering questions about the process. Laser-output power, material and thickness to be cut, percent of maximum thickness to be processed, numbers of hours of operation, duty cycle, and number of lasers, figure into the calculations. Figures 2 and 3 depict the gas supply mode change for pressure versus monthly volume and flowrate versus monthly volume respectively. Once the figures are determined, a system can be chosen with a degree of certainty. As the figures show, many potential gas supply solutions provide a variety of pressures and flows for given monthly volumes. In reality, each system has a broader range of operation than depicted; however, different factors make one system more effective than another in a given application. Gas distributors and users should seek guidance from gas-equipment manufacturers to understand the nuances of each system. Laser-cutting carbon steel with oxygen as the assist gas is typically a low-pressure (150 PSI and lower) and low-flow (less than 1,000 SCFH) application. For example, gas supply systems commonly start with Dura-Cyls liquid cylinders, either exchanged full-for-empty or manifold together, and move to an onsite Perma-Cyl or bulk tank. Pressure or flow requirements do not greatly tax the gas system for these applications. Consider monthly volume and routine system operating costs as key factors for low-pressure and low-flow applications. A clean-cutting process with nitrogen as the assist gas usually requires higher gas pressures (200 PSI and greater) to evacuate the cut zone or kerf of molten material. As shown in Figure 1, a pressure of 200 PSI limits the thickness of material that can be cut to about 0.100 in. For thicker material processing, higher-pressure-liquid systems meet individual gas supply requirements. These high or very high-pressure systems include portable liquid cylinders, such as the Laser-Cyl, and stationary systems, such as the Perma-Cyl VHP, high-pressure bulk tanks, Trifecta, and the HP2 integrated system. All these deliver assist gases higher than 450 PSI with varying net-storage capacities and operating features. Bulk-gas systems become most economical once a laser-cutting facility s volume reaches a certain point. Until recently, only two bulk solutions existed the traditional high-pressure bulk station or Trifecta system combined with a standard-pressure bulk tank. Figure 1. Relationship between material thickness, travel speed, and gas pressure for nitrogen-assist clean cutting.

Page 4 ing flowrates and pressures for advanced laser systems. In addition, uninterrupted service is not possible because of the time required to blowdown, refill, and re-pressurized the tank for filling a major drawback for laser shops working toward maximum beam-on times. Limited net-fill capacities, difficulties in holding and regulating peak pressures, and unwanted vent losses also make the high-pressure bulk tank less desirable for laser applications. Distributors are finding other solutions for providing bulk assist-gas. ConnectFigure 2. Pressure versus monthly volume for optimal gas supply systems. ing the Trifecta, for instance, to a standard-pressure bulk tank lets end users operate without boost pumps to raise the downstream house-line pressure to the level lasers need. The bulk tank acts as a reservoir to feed low-pressure liquid (typically 50 to 200 PSI) to the Trifecta. It supplies sustained flowrates up to 15,000 scfh at pressures up to 450 PSI. The Trifecta also provides an uninterrupted Figure 3. Flow rate versus monthly volume for optimal gas supply systems. Below, the Trifecta supply of high-pressure assystem. sist gas to the laser with little 2 wasted product. Refilling the Today, the HP system is a third bulk tank is done without taking it option. The capabilities of these off-line, which interrupts cutting. high-pressure gas systems vary, so The Trifecta supplies facilities with it is important to choose one that uninterrupted assist gas, and gas dismeets the customer s needs. tributors with a laser solution using a Traditional high-pressure bulk standard-pressure bulk tank. tanks have been the historical workthe HP2, a new, fully integrated horse for laser applications, but this gas supply system, consists of a speis changing. cially designed high-pressure bulk This is because traditional HP tank, control module, and multibulk tanks can t sustain the increasfunction high-pressure vaporizer.

Page 5 Using technology developed and tested over a two-year period, the HP2 supplies pressures and flowrates up to 490 PSI and 12,500 SCFH. The heat-management technology provides capabilities found in no other system, including reduced vent losses, pressure-building times reduced from hours to minutes for example, 100 to 450 PSI in less than 10 minutes increased net-fill capacities, and reduced out-ofservice times for filling. Laser facilities requiring uninterrupted service during the complete filling process can install an optional fill-assist unit. The laser system has become a mainstay as a cutting tool in metal fabrication. Advancements in laser Below, a side view of the Trifecta system. cutting tools have caused other related adaptations. One example: the gas supply system. The higher pressures and flows associated with highpowered faster laser systems have necessitated the development of economical gas solutions capable of providing uninterrupted sources of high-performance assist gas. David Smith is the business manager of the Distribution and Storage Systems Division of Chart Industries, Harrison City, Pa. For more information, contact him at david.smith@chart-ind.com. Copyright 2004 by Penton Media, Inc.