During the 2nd World war, American factories were producing military vehicles and aircraft almost five times faster than other countries.
This was largely due to the great number of innovations by private industry in the field of mass production.
One area of importance was the requirement to cut and join aircraft parts more efficiently. Many factories working on military aircraft adopted a new method of welding that involved the use of an inert gas fed through an electric arc. The engineers discovered that by charging the gas with an electric current created a barrier around the weld, which protected it from oxidation whilst being electrically conductive. This new method, Tig or GTAW welding made for much cleaner welds and much sturdier construction.
In the early 1960’s, another discovery was made. This was that they could boost the TIG welding arc temperatures by speeding up the flow of gas and directing the arc through a constricting nozzle. This method could reach higher temperatures than any other commercial welding machine. The high speed ionized gas, or plasma, conducts electricity from the torch of the power source to the work piece. The plasma heats the work piece, melting the material. The high velocity steam of ionized gas mechanically blows the molten metal away, severing tough metals like a hot knife through butter.
This introduction of what is referred to as plasma arc revolutionized the speed, accuracy and types of cuts manufacturers could make in all types of conductive metals.
A plasma cutting arc can pass through metals with little or less resistance thanks to the unique properties of plasma. There are four states of matter. Most things we deal with in our daily lives are in the form of solids, liquids or gases. The four states are divided based on the way that molecules behave within each one. The solid state of water is ice. Ice is made up of neutrally charged atoms in a hexagonal pattern that forms a solid. Because the molecules stay fairly still relative to each other, they form a solid – something that holds its shape.
Water in its liquid form is its common state. The molecules are still bound to each other, but they are more relative to each other at slow speeds. The liquid has a fixed volume, but no constant shape. It changes to fit whatever container you put it in.
The gas state takes the form of steam. In steam, molecules move around at high speeds, independently of each other. Because the molecules are not bound to each other, a gas has no fixed shape or fixed volume.
If we take the water in solid state ice and apply heat the amount of heat (which translates to the amount of energy) applied to water molecules determines their behavior and therefore their state. Basically, more heat (more energy) excites molecules to the point that they break free of bonds that bind them together. With minimal heat (state 1), the molecules are tightly bound, and you get a solid. With more heat the molecules escape the loose bonds and you get a gas, steam. (state 3)
So what would happen if you were to heat gas even more? This brings us to the fourth state: Plasma.
If you boost a gas to extremely high temperatures, you get the fourth state plasma. The energy begins to break apart the gas molecules and the atoms begin to split. Normal atoms are made up of protons and neutrons in the nucleus, surrounded by a cloud of electrons. In plasma, the electrons separate from the nucleus. Once the energy of heat releases the electrons from the atom, the electrons begin to move around quickly. The electrons are negatively charged and they leave behind their positively charged nuclei. These positively charged nuclei are known as ions.
When fast moving electrons collide with other electrons and ions, they release vast amounts of energy. This energy is what gives plasma its unique status and unbelievable cutting power.
The plasma arc cutting process can be seen in the illustration below. The basis principle is that the arc is formed between the electrode and the work piece through a constricting fine bore, copper nozzle. This increases the speed and temperature of the plasma exiting the nozzle. The temperature of the plasma is in excess of 15000°C and the speed can approach that of sound. This plasma gas flow in conjunction with the high temperature enables a deeply penetrating plasma jet to cut through the work piece material and at the same time molten material is blown away from the cut.
The plasma cutting process is an effective means of cutting both thin and thick materials. Hand-held cutting torches can usually cut up to 50mm thick steel plate, whilst larger water cooled, machine mounted torches can cut steel up to 300mm often used with a mechanized computer-controlled system. Formally, plasma cutters could only work on conductive materials; however, new technologies allow the plasma ignition arc to be enclosed within the nozzle, allowing the cutter to be used for non-conductive work pieces such as glass and plastics (Special conditions apply).