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3. Plasma - gas cutting

Plasma cutting is a kind of arc cutting characterized by the formation of a highly concentrated plasma electric arc, glowing between the electrode and the material which melts it and propels from the cutting gap. Plasma arc is a highly ionized gas, moving from the plasma jet with a speed close to the speed of sound.

The temperature of plasma beam oscillates at values of 10 000-30 000 K and depends on the current and the type and composition of the plasma gas. Due to the high heat of the plasma arc, cutting can be performed in a wide speed range. However, it decides about the quality of the process and with increasing speed the quality drops, the width of the gap decreases and eventually it reaches the point where there is no cutting. Depending on the material to be cut various plasma gases are used. The power source is a plasma generator. During the cutting, the torch is maintained in perpendicular position or at a right angle (during gougingor bevelling) to the surface of the object being cut and at a constant distance from it.Burners, in which the plasma gas is air, are equipped with special electrodes resistant to oxidation.The electrodes of hafnium are used, embedded in a copper body, providing intense heat dissipation.

This type of cut allows the separation of all electrically conductive materials. The process of plasma cutting is carried out by hand or in mechanized or robotic manner.Along with oxygen cutting it dominated the industry - about 90% of cutting machines sold on the world market are devices for plasma and oxygen cutting.
Unquestionable advantages of plasma burners include high cutting speed and undistorted cut elements.Due to the high temperature of plasma arc, cutting begins immediately, without heating.The disadvantages of this process, however, are a high noise level, a large amount of gases and fumes and the risk of electric shock.

Maximum sheet size: 3000 x 13000 mm
Cutting thickness: carbon steel from 6 to 300mm

The use of plasma arc allowing the cutting of all electrically conductive materials found application in the industry in the 50s of the last century.The development of cutting technology and specialized devices allowed extending the range of applications of plasma cutting with the previously developed cutting stainless steel and light alloys with low-alloy steels.

The huge development of this technology meant that under certain conditions, plasma cutting has become competitive for processes such as e.g. laser cutting or gas cutting.Furthermore, depending on the production volume and the needs of achieved surface quality and reproducibility of the dimensions of the thermal cutting processes can be carried out manually, semi-automatically and automatically (CNC).Plasma-creatinggas, flowing through the electric arc glowing between the electrodes is ionized and due to the high power density produces a stream of plasma (ionized gas). The nozzle mounted in the burner focuses the plasma arc. Cooled wall nozzle causes narrowing of the arch.High temperature in the core of the plasma arc and a very high speed of plasma jet (kinetic energy) is a phenomenon causing materialbeing melted and blown out of the gap. Plasma gas must be of high enthalpy and high thermal conductivity as well as smallest possible dissociation and ionization potential and large molecular weight.

For plasma cutting are used, among others:
- argon - excellent gas component to striking and maintaining an arc used in admixture with hydrogen,
- hydrogen–for cutting austenitic stainless steel and light alloys, used in conjunction with argon or nitrogen,
- nitrogen - provides cutting at high speeds without the oxidation of the edges and reduces the formation of overhangs,
- oxygen - used for efficient cutting of low alloy steels guarantees a smooth surface of the material being cut, free from overhangs and oxides.
Cutting operations can be performed under a layer of water, which causes a significant reduction in noise levels. Plasma cutting process involves melting the metal and disposing of the cutting gap with highly concentrated plasma arc, glowing between tungsten and the object being cut.Plasma arc is a highly ionized gas with high energy, moving from the plasma jet toward a narrow cutting gap, at the speedclose to the speed of sound. The temperature of the plasma jet depends on the current intensity, the degree of constriction of the arc and the nature and composition of the plasma gas, and is in the range of 10 000 - 30 000 K.For plasma cutting there is used only DC current of negative polarity supplied from a rectifier or inverter power sources.It is possible to cut all conductive structural materials.The process of plasma cutting is used for manual, mechanized and robotic cutting of steel and non-ferrous metals at high speeds.Due to the high temperature of plasma arc, cutting starts immediately, without heating.

The advantages of plasma cutting include:
- significant smoothness of cut surfaces,
- reliable and repeatable ignition of pilot arc regardless of the coverage of the cut material with the layer of rust or paint,
- prolonged life of consumable elements through the use of appropriate systems,
- Fully automated cutting with machine systems,

Basic parameters of plasma cutting are:
- current in A,
- arc voltage in V,
- cutting speed in m / min,
- the type and pressure in MPa (bar) and gas flow of the plasma in l / min,
- the type and design of the electrode,
- a convergent nozzle diameter in mm,
- the position of the torch relative to the object to be cut.

With manual plasma cutting , the operator governs only the cutting speed and the distance between the nozzle and the cut the object, while the other parameters are fixed, maintained by control system of the device on the level set by the operator. Current determines the temperature and energy of the plasma arc.This implies the fact that when the current increases, the cutting speed increases at the same time or at a given cutting speed it is possible to cut thicker materials,however, the durability of the electrodes significantly decreases.Excessive current causes decreasing of cutting quality, increasing of the width of the gap, occurring of rounding of the upper edge and the deviations from perpendicularity.Too small current initially causes the appearance of the metal overhangs at the lower edge, thenthere is no intersection.Plasma arc voltage determines the smooth running of the process of plasma cutting, and therefore must be carefully controlled. Depending on the current, the arc voltage, because of the very high degree of concentration of the plasmaof arc, equals 50 200 V.The power sources must therefore have power of idle running approx. 150 400 V. Due to the high heat of the plasma arc the cutting process may be conducted within relatively wide range of cutting speeds.

The cutting speed determines the quality of the cutting, especially in the case of manual cutting.When cutting speed increases, decreases cutting quality and the width of the cut; difficult to remove overhang of metal at the bottom edge appears and ultimately lack of the intersection may be observed.Too low cutting speed leads to an increase of the width of cuttinggap and rounding of the upper edge, wider at the top than at the bottom of the gap, and the appearance of the metal overhang at the lower edge. The flow velocity of the plasma jet from the burner and its temperature depend on the intensity of the current, the diameter and the shape of the nozzle as well as the distance between the torch and the cut object, but also on the type of plasma gas and its pressure.Depending on the type of material being cut there are various plasma gases used. These are mostly oxygen, air, nitrogen, argon and mixtures of argon with hydrogen and nitrogen with hydrogen. In the first plasma cutting equipment was used only argon and argon- hydrogen mixtures.Due to the high price of these gases, the development of plasma cutting was aimed not only to increase the quality and speed of cutting, but also to replace argon with much cheaper gases.Initially, it was nitrogen, and later air and oxygen, givingsimilar cutting speeds with smaller currents.

Plasma cutting can be carried out manually as well as can be mechanized, automated and robotized in all positions.Manual welding, thanks to the low weight of the burner and inverter sources of power, can be used under mounting conditions in tight spaces.There are also built devices that enable robotic cutting of structures under water at great depths.

Technology and technique of plasma cutting are dependent mainly on the design of the burner and the basic conditions for technological cutting shall often be determined on the basis of recommendations or directories of the manufacturer.Plasma cutting torches are used at a current strength of 30 40 A, 30 100 A, and high-power, 100 1000 A.Torches for manual cutting are typically air-cooled, and the current of plasma arc is lower than 100 A.High power burners are used for mechanized cutting controlled numerically and require water cooling.

Modern structures of burners have self-cantering nozzles and electrodes, in order to ensure their greatest possible durability.There are numerous designs of plasma torches which enable increase of the quality and cutting speeds while reducing cutting costs. Plasma gas melts and blows the molten metal out of the cut, and the shielding gas shields the cutting area from air and additionally cools it.

Using additional narrowing of the plasma arc with a protective gas increases the degree of plasma narrowing and its temperature, which can increase the velocity of cutting at the same current parameters.In this embodiment, the plasma gas is typically argon or a mixture of H2, Ar or N2, Ar, and the second gas type, the protective and narrowing one, depends on the type of metal to be cut.For example, when cutting low carbon steels and low-alloy steels the second gas may be air or oxygen, which provides an additional increase in cutting speed due to the exothermic combustion of iron.When cutting stainless steels and aluminum, the second gas may be nitrogen.