Plasma cutting

Workpieces made of electrically conductive materials are cut through by using an accelerated jet of hot plasma. It is an effective way to cut thick sheet metal.

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Whether you are creating artwork or manufacturing finished parts, plasma cutting offers unlimited possibilities for cutting aluminium, stainless steel and more. But what exactly is behind this relatively new technology? We clarify the most important questions in our brief overview with the most important facts about plasma cutters and plasma cutting.

Plasma cutting is a process in which electrically conductive materials are cut through by means of an accelerated jet of hot plasma. Typical materials that can be cut with a plasma torch are steel, stainless steel, aluminium, brass, copper and other conductive metals. Plasma cutting is widely used in manufacturing, automotive repair and restoration, industrial construction, salvage and scrapping. Due to the high speed and precision of the cuts at low cost, plasma cutting is widely used from large industrial CNC applications to small hobby companies where the materials are subsequently used for welding. Plasma cutting - Conductive gas with a temperature of up to 30,000°C makes plasma cutting so special.

The basic process in plasma cutting and welding is to create an electrical channel of superheated, electrically ionised gas – i.e. plasma - from the plasma cutter itself through the workpiece to be cut, thus forming a finished circuit back to the plasma cutter via an earth terminal. This is achieved by a compressed gas (oxygen, air, inert gas and others depending on the material to be cut) which is blown to the workpiece at high speed through a focused nozzle. Within the gas, an arc forms between an electrode near the gas nozzle and the workpiece itself. This electric arc ionises part of the gas and creates an electrically conductive plasma channel. As the current from the cutting torch of the plasma cutter flows through this plasma, it gives off enough heat to melt through the workpiece. At the same time, much of the high-speed plasma and compressed gas blows the hot molten metal away, separating the workpiece.

Plasma cutting is an effective way to cut thin and thick materials. Hand torches can usually cut up to 38 mm thick steel sheet, stronger computer controlled torches can cut up to 150 mm thick steel sheet. Since plasma cutters produce a very hot and very localised “cone” for cutting, they are very useful for cutting and welding sheets in curved or angled shapes.

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Manual plasma cutters are generally used by workshops for thin metal processing, factory maintenance, agricultural maintenance, welding repair centres, metal service centres (scrap, welding and dismantling), construction work (e.g. buildings and bridges), commercial shipbuilding, trailer production, car repair and works of art (manufacturing and welding).

Mechanised plasma cutters are usually much larger than manual plasma cutters and are used in conjunction with cutting tables. Mechanised plasma cutters can be integrated into a punching, laser or robot cutting system. The size of a mechanised plasma cutter depends on the table and portal used. These systems are not easy to manoeuvre, so all their components should be considered along with the layout of the system prior to installation.

Meanwhile, manufacturers also offer combination units that are suitable for both plasma cutting and welding. In the industrial sector, the rule of thumb is: the more complex the requirements for plasma cutting, the higher the costs.

Plasma cutting emerged from plasma welding in the1960s and developed into a very productive process for cutting sheet metal and plates in the 1980s. Compared to traditional “metal vs. metal” cutting, plasma cutting does not produce metal chips and offers precise cuts. The early plasma cutters were large, slow and expensive. Therefore they were mainly used for the repetition of cutting patterns in mass production mode. As with other machine tools, CNC (Computer Numerical Control) technology was used in the plasma cutter from the late 1980s to the 1990s. Thanks to CNC technology, plasma cutters were given greater flexibility in cutting different shapes based on a series of various instructions programmed into the numerical control of the machine. However, CNC plasma cutting machines were usually limited to cutting patterns and parts from flat steel sheets with only two axes of movement.

In the last ten years, the manufacturers of the various plasma cutters have developed completely new models with a smaller nozzle and a thinner plasma arc. This enables laser-like precision at the plasma cutting edges. Several manufacturers have combined CNC precision control with these torches to produce parts that require little or no rework, simplifying other processes such as welding.

The term “thermal separation” is used as an umbrella term for processes in which materials are cut or formed by the action of heat with or without cutting oxygen flow in such a way that no reworking is necessary in further processing. The three dominant processes are Oxy-fuel, plasma and laser cutting.

Oxy-fuel cutting

When hydrocarbons are oxidised, they generate heat. As with other combustion processes, oxy-fuel cutting does not require expensive equipment, the energy source is easy to transport and most processes require neither electricity nor cooling water. A burner and a fuel gas bottle are usually sufficient. Oxy-fuel cutting is the predominant process for cutting heavy, unalloyed and low-alloy steel and is also used to prepare the material for subsequent welding. After the autogenous flame has brought the material to ignition temperature, the oxygen jet is switched on and causes the material to burn. How quickly the ignition temperature is reached depends on the fuel gas. The speed for the correct cut depends on the purity of the oxygen and the speed of the oxygen gas jet. High purity oxygen, optimised nozzle design and correct fuel gas guarantee high productivity and minimise overall process costs.

Plasma cutting

Plasma cutting was developed in the 1950s to cut metals that could not be fired (e.g. stainless steels, aluminium and copper). In plasma cutting, the gas in the nozzle is ionised and focused by the special design of the nozzle. Only with this hot plasma stream materials such as plastics (without transferred arc) can be cut. With metal materials, plasma cutting also ignites an arc between the electrode and the workpiece to increase energy transfer. A very narrow nozzle opening focuses the arc and the plasma current. An additional lacing of the discharge path can be achieved by a secondary gas (protective gas). The choice of the right plasma/protective gas combination can significantly reduce the overall process costs.

Laser cutting

Laser cutting is the latest thermal cutting technology and was developed after plasma cutting. The laser beam is generated in the resonator cavity of the laser cutting system. While the consumption of the resonator gas is low, its purity and the correct composition are decisive. Special resonator gases protect the devices from the cylinder into the resonator cavity and optimise the cutting performance. For cutting and welding, the laser beam is guided from the resonator to the cutting head through the beam path system. It must be ensured that the system is free of solvents, particles and vapors. Especially for high performance systems (> 4kW) nitrogen from a liquid source is recommended. In laser cutting, oxygen or nitrogen can serve as cutting gas. Oxygen is used for unalloyed and low-alloy steel, although the process is similar to oxy-fuel cutting. Here, too, the purity of the oxygen plays an important role. Nitrogen is used for stainless steel, aluminium and nickel alloys to achieve a clean edge and maintain the critical properties of the base material.

Water is used as a coolant in many industrial processes that bring high temperatures to the process. The same applies to water injection in plasma cutting. Water is injected through the injector into the plasma arc of the plasma cutter. The plasma arc is usually created when nitrogen is used as plasma gas, as is the case with the majority of plasma cutters. As soon as the water is injected into the plasma arc, this leads to a high constriction. In this special process, the temperature rises significantly to 30,000°C and above. If one compares the process advantages mentioned above with conventional plasma, it can be seen that both the cutting quality and the rectangularity of the cut are significantly improved and the materials are ideally prepared for welding. In addition to the improvement in cutting quality during plasma cutting, an increase in cutting speed, a lower risk of double curvature and a reduction in nozzle erosion can also be observed.

Swirl gas is often used in the plasma cutting industry to achieve better containment of the plasma column and a more stable necking arc. As the number of inlet gas vortices increases, the centrifugal force moves the maximum pressure point to the edge of the plenum and the minimum pressure point much closer to the axis. The difference between the maximum and minimum pressure increases with the number of swirls. The large pressure difference in the radial direction narrows the arc and leads to high current density and ohm heating near the axis.

This leads to a much higher temperature near the cathode. It should be noted that twist gas accelerates cathode erosion for two reasons: Increasing the pressure in the plenum and changing the flow pattern near the cathode. It should also be taken into account that the gas with high swirl number increases the swirl velocity component at the cutting point according to the conservation of angular momentum. It is assumed that this causes different angles at the left and right edges of the kerf.

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