Plasma cutting (plasma arc cutting) is a melting process in which a jet of ionised gas at temperatures above 20,000°C is used to melt and expel material from the cut. During the process, an electric arc is struck between an electrode (cathode) and the workpiece (anode). The electrode is recessed in a water- or air-cooled gas nozzle which constricts the arc causing the narrow, high temperature, high velocity plasma jet to form.
When the plasma jet hits the workpiece, recombination takes place and the gas reverts to its normal state, emitting intense heat as it does so. This heat melts the metal and the gas flow ejects it from the cut. Plasma gases are usually argon, argon/hydrogen or nitrogen. These inert gases can be replaced by air but this requires a special electrode of hafnium or zirconium. Use of compressed air makes this variant of the plasma process highly competitive with the oxy-fuel process for cutting carbon-manganese and stainless steels up to 20mm thick. Inert gases are preferred for high quality cuts in reactive alloys.
Plasma arc can cut a very wide range of electrically conductive alloys including plain carbon and stainless steels, aluminium and its alloys, nickel alloys and titanium. The method was originally developed to cut materials which could not be satisfactorily cut by the oxy-fuel process. Normally, the component or sheet to be cut remains stationary and the plasma torch is moved. Additionally, because the cost of the plasma torch is low compared with the price of the manipulation equipment, it is common to fit several torches to a cutting table.
Plasma arc cutting can also be carried out under water using specialised equipment.
High tolerance plasma arc cutting (HTPAC) is an important development of plasma arc technology. The process gives better precision on material under 12mm thick and can be a low-cost alternative to laser cutting.
More information on plasma cutting is available in Profile cutting - guide to process selection.