Plasma cutting

Plasma is an ionized gas which is electrically conducive at high temperature containing both the negative and the positive particles plus the generated neutral molecules and atoms .Plasma cutting is a thermic cutting process where a plasma arc is used. This technology has been used since the 1950's for the cutting of metals. The basic principle is that the arc formed between the electrode and the workpiece is constricted by a fine bore, copper nozzle. This increases the temperature and velocity of the plasma emanating from the nozzle. The temperature of the plasma is in excess of 20 000°C and the velocity can approach the speed of sound. When used for cutting, the plasma gas flow is increased so that the deeply penetrating plasma jet cuts through the material and molten material is removed in the efflux plasma.

There are two variants of plasma cutting: plasma beam cutting and plasm arc cutting. This latter technology is the most wide-spread.

As there is no heat generated during the process, the material to be cut does not get burned. On initiation, the pilot arc is formed within the body of the torch between the electrode and the nozzle. For cutting, the arc must be transferred to the work piece in the so-called 'transferred' arc mode. The electrode has a negative polarity and the work piece a positive polarity so that the majority of the arc energy (approximately two thirds) is used for cutting. The energy required to melt the material is partly generated by the plasma beam and partly by the electrical arc.

Plasma is a thermally high-heated, electrically conductive gas. That means that neutral atoms dissolve into ions and electrons by adding ionisation energy. This energy can be produced by very high temperatures or strong electric fields. Generally, plasma reacts like a gas with a neutral outside effect.

Plasma cutting is a thermal fusion cutting method which is realised with an electric arc constricted by a nozzle. The cutting process starts first with a pilot arc which is ignited between nozzle and electrode (cathode) by high voltage. The pilot arc has low energy and ionises partly the atoms between plasma torch and work piece. As soon as the pilot arc touches the work piece, the electric circuit closes and the main are is ignited by the increased power. The high thermal energy of the arc and the high kinetic energy of the plasma gas melt the material and the molten material is driven out of the kerf. The small heat affected zone and high cutting speeds are particularly huge advantages of this method.

The main technical characteristics of plasma cutting: material thickness; nozzle type; gas composition; voltage; current intensity; the size and distance of the wolfram electrode from the surface of the nozzle; the distance of the nozzle from the cutting surface; the direction and speed of cutting.

Plasma- and shield gases

These days there are many variants for the application of the gases and gas components and their ratios. These gases are applied during the process in accordance with their characteristics. The type of gas used depends on the cutting quality.

Plasma gases

• Air – most frequently used gas
• Oxygen (O₂) - ensures the highest quality and speed when cutting carbon steels
• Nitrogen(N2) – best for cutting aluminium and non-corrosive materials
• Argon - Hydrogen gas mixture Ar (65%) - H2 (35%) - perfect for cutting aluminium and non-corrosive materials
• Nitrogen – Hydrogen gas mixture N2 (95%) - H2 (5%) - used for non-corrosive steels

Shield gases

• Air
• Nitrogen N2
• Carbon-dioxide CO2
• Instead of shield gases the application of water is wide-spread, as well.

• Traditional plasma cutting without shield gases
  This type of cutting involves usually only one gas, mainly air or nitrogen. These systems are mainly used for manual plasma cutting devices.
   
• Dual gas plasma cutting
  The process operates basically in the same manner as the conventional system but a secondary gas shield is introduced around the nozzle. The beneficial effects of the secondary gas are increased arc constriction and more effective 'blowing away' of the dross. The plasma forming gas is normally argon, argon-H2 or nitrogen and the secondary gas is selected according to the metal being cut.
   
• Water shield plasma cutting
  A modified version of dual gas plasma cutting where water is used for the surface protection of the corrosive materials instead of a shield gas. Water ensures the better cooling of the work piece resulting in a better cutting surface. It can only be used with machine cutting due to the fume and steam generated during the process.
   
• Water injection plasma cutting
  This process involves oxygen ( when cutting carbon steels) or nitrogen (for aluminium and non-corrosive materials) used as a plasma gas and water is injected directly to the plasma arc. Due to the water injection the plasma arc gets even more concentrated, as its diameter reduces and the result is an even higher cutting speed. A small amount of the water evaporates but the residue cools down the cutting surface which gives a more precise cutting surface. This process can be used for carbon steels, as well, not only for aluminium or non-corrosive materials.
   
• Precision plasma cutting
  In an attempt to improve cut quality and to compete with the superior cut quality of laser systems, precision plasma arc cutting systems are available which operate with a highly constricted plasma. Focusing of the plasma is effected by forcing the oxygen generated plasma to swirl as it enters the plasma orifice and a secondary flow of gas is injected downstream of the plasma nozzle