Plasma systems today are available with power levels from 30 to over 800 amps, both in hand held and mechanized system configurations. A long term engineering evolution has refined the technology, ultimately splitting it into three basic types of plasma cutters that fit virtually every facet of metal fabricators needs.
As I’m often involved in internet-based metal fabricating user group discussions that tend to revolve around a variety of metal fabricating processes, I often hear comments that regard plasma cutting (in general) as a commodity type process.
What I mean by this is that many people involved in the metal-fabricating business think that all plasma cutting systems produce the same type of end result. I hear comments along the lines of, “CNC plasma cut holes will be out of round and will have serious tolerance issues, plasma cut edges will have increased hardess and a large heat affected zone and plasma cutting systems use a lot of consumable parts, which make it a costly process.” When I run into these types of comments, I spend as much time as possible trying to educate those making the comments in regards to today’s level of plasma cutting technology.
In reality, there are quite a few manufacturers of plasma cutting systems. There are hand-held air-plasma systems, along with plasma systems designed strictly for mechanized cutting applications. There are plasma cutters the size of a two slice toaster that can run on ordinary 120-V household current and can sever 0.5 in. steel and there are 800-amp mechanized-plasma systems that can cut 6.25 in. stainless steel!
To describe the technology known as high-definition plasma, I’ll review the three distinct types of plasma-cutting systems found on the market today, produced by a few different manufacturers. The three major types of plasma cutting systems are: air plasma, conventional-oxygen plasma and high-definition class plasma. Let’s look at the focus and technology of each.
This class of plasma systems has evolved from some of the first lower cost plasma cutting systems that could be used with hand held torches. Hand plasma cutting was first developed in the mid 1960s. At that time these machines were simply mechanized-plasma cutting systems that had a special torch designed for cutting by hand.
The process involved a large, up to 400-amp power supply that required three-phase industrial power. The torch was much larger in size and weight compared to the industry standard oxy-acetylene hand torch. The process was not for the meek. There was high voltage, a lot of molten-metal sparks, and it gave a very poor cut quality with low (expensive) consumable life. These torches used nitrogen as the plasma gas and carbon dioxide as the shield gas.
Regular compressed air was eventually used as a plasma gas in place of nitrogen and newer power supply technology allowed for more compact, lower cost systems that could run off single-phase power. The early air plasma’s, pioneered by companies like Thermal Dynamics, Hypertherm and Esab, had more compact torches and could operate from the power available in small shops and garages. These systems used a lot of consumable parts, but there was finally a portable solution for cutting steel, stainless and aluminum, something that the industry standard oxy-acetylene couldn’t do.
Today’s state-of-the-art air-plasma systems involve inverter-based power supplies (compact, light, efficient, reliable), compact, easy-to-use hand-held torches (some of the major manufacturers also offer mechanized torches for these systems) and provide a very nice cut quality at reasonable speeds with relatively low operating costs. From a cut quality perspective, an air-plasma torch is as good as the person doing the cutting. With a shaky hand, you’ll get a rough cut edge. When it’s mounted on a precision cutting machine, expect a very smooth, relatively accurate cut edge with tolerances in the plus or minus 0.030 in. range. Air plasma cutting will create a nitride case hardened edge (about 0.006 in. heat affected zone) on steel, and will provide some oxidation on stainless and aluminum.
The air-plasma torches (for the most part) are air cooled. Consumable life varies from manufacturer to manufacturer depending on their level of torch technology and design. With most of the major brands of air plasma, cutting costs are low, consumable life is good, and reliability and portability are excellent.
Air plasma summary
Since there are extremely high numbers of air plasma systems in the world, likely due to widespread use, portability and low cost, many in the industry link the cut quality of air plasma systems to all plasma systems.
Conventional oxygen plasma
Conventional-oxygen-based plasma systems evolved as well from the mechanized plasma cutting systems from the early 1960s. The early systems generally used nitrogen and carbon dioxide as the plasma and shield gases respectively and used liquid cooled torches to enhance consumable life. The early mechanized-plasma systems as produced up through the mid 1980s worked well and were very productive for cutting stainless steel and aluminum up through about 6 in. thicknesses. However steel cutting was limited to less than 0.625 in. and cut quality was poor.
The use of nitrogen as the plasma gas produced a very hard, nitride edge on carbon steels, and when thicknesses above 0.625 in. were cut, a heavy, re-welded dross (re-solidified steel) occurred often on the bottom edge. Further, the cut edge typically would have a four-to-six-degree taper, and there was a lot of top edge rounding on the cut. Still, plasma was a very interesting process in mechanized cutting applications due to its high speed productivity. A single-plasma torch cutting 0.5 in. steel could cut as many parts per shift as six oxy-acetylene torches.
Because of the productivity demands for mechanized plasma cutters, a few of the manufacturers worked hard to develop new systems and produce improvements in cut quality, consumable life, cut speeds and thickness capacity. The real drive was to further increase thickness and cut quality on steel, since steel made up over 95 percent of the metals used in the fabrication industry.
Around 1982, Hypertherm developed and introduced the first commercially successful oxygen-plasma cutting system. By using oxygen as the plasma gas and water injection to help constrict the arc (increasing energy density), steel up to 1 in. thick could be cut with clean, dross-free edges, no top edge rounding and minimal edge taper. Furthermore, cut speeds were increased while cutting power (amperage) was decreased, resulting in lower utility cost, less noise and ultraviolet glare on the shop floor. There was a drawback however. Consumable life decreased due to the use of oxygen as the plasma gas and its effect in the high temperature plasma-torch environment.
A few years later (early 1990s) another development in the engineering labs produced newer technology that dramatically improved consumable life when cutting with oxygen. In fact, consumable life was increased five to six times, which further reduced the cost of oxygen-plasma cutting.
The use of oxygen as the plasma gas dramatically increased acceptance of mechanized plasma cutting systems for use with steel applications to 1 in. The introduction of the long-life-oxygen technology further bolstered its use. Further developments have allowed oxygen plasma cutting technology to cut up to a 2 in. steel thickness. Many of these same systems can also cut up to 6.25 in. thickness for stainless and aluminum when using gases such as argon/hydrogen and can hold tolerances in the +/- 0.020 in. range in many applications.
High-definition class plasma
Engineers that were bending the laws of high-temperature physics and developing plasma systems for many years had discovered that consumable life was a balancing act. The choice was best cut quality or best consumable parts life. By increasing the energy density (energy density is determined by the orifice size of the plasma nozzle and can be measured as amps per square inch) of a plasma cutting process, the edge of the cut would become more square, the dross on the bottom edge would minimize and the kerf width would become narrower. Unfortunately, when these benefits occurred, the life of the torch electrode and nozzle would drop dramatically! The process-design engineers were forced to cut back a bit on cut quality to design cutting systems that had a reasonable cost per foot of cut.
When the long-life oxygen technology was developed, a process that used an advanced power supply and gas flow control technology, plasma-process engineers revisited methods of improving cut quality by increasing plasma-arc energy density. In the early 1990s, the first high-definition plasma system was introduced. Essentially, by developing a unique vented two-piece nozzle design with a primary and secondary constricting orifice, the energy density of an oxygen-plasma arc was increased from 15,000 amps per square inch to around 60,000 amps per square inch.
This resulting energy increase allowed squarer cut edges, cleaner dross free cuts, a narrower kerf and excellent consumable-parts life. The technology required further refinements in power-supply current control along with gas mixing and flow control to the torch. The torch itself has a few engineering innovations involving the vented-nozzle design and advanced liquid cooling very close to the nozzle orifice.
The first high-definition systems were limited to 70 amps output and a maximum cut thickness of 0.375 in. for steel. Today there are high-definition systems that can cut with oxygen up to a 3 in. thickness at 400 amps on carbon steel with better cut quality and faster speeds compared to any other process.
High-definition plasma summary
Today’s high definition systems allow very high levels of automation. With the most advanced systems virtually all of the machine operators expertise (required to get good cut quality on earlier plasma systems) is essentially canned in the CAM software that manages the day-to-day cutting operations. Cut holes are round, have virtually no taper. Edges are square and dross free. Cut-to-cut cycle times allow for much higher levels of productivity. A single-plasma system can cut material thicknesses from thin gauge to over 6 in., using the same torch. The torch can cut and mark the plate through he same nozzle orifice. It’s not uncommon with these systems to achieve cut part accuracies within the +/- 0.010 in. range.
High-definition plasma technology has breathed new life into a 50-year-old technology.