The plasma cutting method is the most universal of the three methods presented here. A high-energy plasma jet is used for the local melting of the material to be cut. Inside the torch geometry a previously neutral gas is converted to hot plasma by electrical ionisation, which emerges with high kinetic energy as a burning electrical arc between the electrode and workpiece through a narrow, mostly water-cooled, nozzle orifice at the front end of the torch. By plasma-physical transformation, the cutting plasma reaches temperatures of over 30,000 ° C.
As plasma gas essentially nitrogen, nitrogen-hydrogen mixtures, argon-hydrogen mixtures but also very often simple compressed air are used. Suitable for cutting with plasma processes are all electrically conductive materials; with a special variation of this process, also non-conductive insulating materials like plastics can be separated.
The hot plasma beam melts the material and drives the non-evaporated portion of the melt out of the joint, which is formed by the forward movement of the plasma torch above the workpiece. The process is by its large number of parameters (cutting current, nozzle orifice, cutting speed, gas composition, distance of the plasma torch to the workpiece, etc.) individually adjustable. However, the method also requires experienced operators because of its high complexity.
Plasma cutting is often manually used in a simple form, and can be due to its electrical actuating variables excellently controlled and parameterized so that the automated use at CNC cutting portals, particularly in the cutting current range above 100A, is the norm.
The manual plasma cutting is widely used in scrap separation, chemical plants decommissioning and in crafts as well as in small batch production and shipbuilding. Manual and automated, the process is applied to surface gouging of worn functional layers as preparation for regeneration. The automated variant can be found in general wherever any kind of metallic materials are cut as semi-finished products, such as in shipyards, container construction, general steel construction and heavy mechanical engineering.
The Laser Beam
The laser beam is applied to separate almost all materials, e.g. steels, non-ferrous metals, plastics, ceramics, wood ... .However, the economic thickness limit is at a maximum of 50 mm for high-alloy steels and 25 mm for non- and lowalloy steels as well as non-ferrous metals. CO2 gas lasers as well as disk-, fiber- and diode-solid-state lasers are used for the beam generation. In industrial applications three different process variants are used depending on the material to be cut.
The laser beam cutting process uses – as the oxy-fuel cutting process – a beam of cutting oxygen for the transformation of non- and low materials into highly viscous dross. The combustion of the material and blow off of the dross out of the cut is carried out analogously to the flame cutting process, only preheating of the material to ignition temperature is achieved by means of laser energy.
The laser beam fusion cutting melts, in analogy with the plasma cutting process, the material (high-alloy steels and non-ferrous metals) over its entire thickness, however, uses for this purpose, in contrast to the plasma arc, the laser beam energy. The liquefied material is by means of gas pressure (inert nitrogen, rarely also inert gas) blown out of the kerf. Beneficial for cuts on highalloy steels is the formation of smooth cut edges by the inert cutting gas.
In plastics and organic materials, the variant of the laser beam sublimation cutting is used. The high energy density of the laser beam evaporates (sublimates) the material. The resulting vapour pressure drives the vaporized material, often with the support of an inert cutting gas (nitrogen), from the kerf.
The laser beam cutting process and its variants are used in highly automated manufacturing processes, in all areas with high requirements in terms of cutting quality and precision. The effort is worth despite the very high capital and energy costs, due to the very moderate costs for wearing parts compared to plasma, and the almost entirely attributable rework.
This kind of thermal cutting is used for greater thickness of cutting materials, in practice from 15mm. In area of 35mm thickness, there is no other technology except Oxy-fuel which can be used.
When oxy-fuel cutting, an oxy-fuel flame is used as a heat source. Acetylene, propane, natural gas or mixtures thereof may be used as fuel gas in this cutting process. To start the process, the material to be cut is heated to ignition temperature by means of a heating or preheating flame, and at the same time the surface is cleaned of any impurities such as rust, scale, etc. When the ignition temperature is reached, the oxy-fuel jet is switched on and an exothermic reaction burns the material along the gas jet into the depth of the workpiece and in the advance direction of the beam.
In this process, the material is burned to a highly fluid low-melting dross, which is blown out at the bottom of the kerf by the pressure of the cutting jet. The decisive factor for this method is that the ignition temperature of the material and the melting temperature of its dross are lower than the melting temperature of the material per se. In addition, essential to the process is that the dross has a low viscosity (fluid) and that the thermal conductivity of the material to be cut is as low as possible. For example, aluminum and copper alloys are unsuitable for the oxyfuel cutting process, whereas mild steel, low alloy steel, cast steel and titanium have a very good suitability for this separation process.
The achievable cutting speed depends on the materiál, the thickness to be cut and kind of fuel gas. At optimum adjustment, the spark jet emerges vertically on the underside of the workpiece.
The manual oxy-fuel cutting is widely used in scrap separation, dismantling of industrial plants, and crafts and shipbuilding. The automated version can be found in general wherever ferrous materials are cut as semifinished products, such as in shipyards, general steel construction and heavy mechanical engineering.