Metal cutting is one of the more commonly known branches of metalworking, and there are two main types of metal cutting – mechanical cutting and thermal cutting. As the name suggests, the main difference between thermal and mechanical cutting is the appliance used to perform the cutting process.
Both mechanical and thermal cutting consists of multiple possible options for the actual types of metal cutting. For example, plasma cutting falls under the category of thermal cutting since the process itself is all about using an ionized gas to cut metal.
The plasma cutting process can be used to cut metal of many different thicknesses, ranging from relatively thin sheet metal to thick metal plates. As with any other method or approach to metal cutting, plasma cutting has its benefits and problems. Nevertheless, the nature of this topic requires us to go through what plasma is in the first place.
Plasma is the fourth state of matter, different from the three commonly known ones – liquid, solid, and gas. Plasma can be found naturally in the upper atmosphere of our planet. It is also a part of high-temperature fire and lighting. Both the lightning and the atmosphere are not precisely the best places to try and collect plasma from – which is why the thermal method is the one used to perform plasma cutting.
Plasma can be acquired by using intense heat on gas, where another definition of plasma comes from. That definition describes plasma as “an ionized gas-like substance that conducts electricity.” Plasma is not only used for industrial purposes, either – it has been a part of anyone’s lives for a long time, with fluorescent lamps, neon signs, and plasma TVs being some of the more commonly known examples.
The plasma cutting process on its own is not particularly complicated – the main tool forces some gas into the nozzle. It transforms it into plasma before making contact with the metal piece. The transformation is done using an electric current strong enough to trigger the ionization process of the gas, transforming it into plasma.
The plasma cutting process uses an electric current to produce the plasma is the main reason this exact process is often referred to as the plasma arc cutting process. As for the cutting of the metal piece with said plasma – two significant parts of this process differ a lot (melting the metal piece and blowing away the resulting molten metal).
If we’re talking specifically about using plasma to melt the metal piece, there is a dedicated appliance for plasma cutting, and it consists of three main parts:
- Power supply. As the name suggests, the primary purpose of a power supply is to provide the energy necessary for the plasma arc to stay consistent after the ionization process has been initiated. There are some variations to power suppliers in plasma cutting, but the more common ones always range between 240 and 400 Volts of Direct Current (or VDC, for short).
- Circuit. As we’ve mentioned before, the ionization process is triggered by an electric current – and the primary purpose of a starting circuit is to provide that current, a high-intensity electric arc to initiate the ionization process for the flowing gas. The arch itself can be initiated either by using a “blowback start” or with a moving electrode, and the voltage of this current usually rests at 5000-10000 volts at ~2 MHz.
- Torch. Last but not least here, the torch is a part of this appliance that holds both the nozzle and the electrode in place. It is also relatively common for the torch to be used for cooling purposes since the entire appliance reaches extremely high temperatures while working for obvious reasons.
That is not to say that this kind of appliance does not have any other additions to it – there are quite a lot of variations in plasma cutting, and some of them tend to improve the results of the process to an impressive degree.
For example, it is possible to inject water into the plasma arc generated during the plasma cutting process – and this variation of plasma cutting is called water injection. The injection of water into this process as a whole generates a number of improvements over the classic plasma arc cutting process – such as higher peak plasma temperature, reduced nozzle erosion, higher cutting speed, as well as better cutting quality.
Since we have started with water already, it is fair to mention that it does not have to be injected directly into the plasma stream – this method is called a water shroud. It can be used to reduce the overall noise while improving the lifetime of a plasma cutting nozzle.
As an alternative, the metal piece can be submerged in up to 60 mm of water to provide the same effect. However, since this method does not interact with the constriction levels in any way, there are little to no performance improvements that can be gained with this type of plasma cutting.
Besides injecting water, there are other methods of plasma stream constriction. A dual gas method uses a secondary gas stream for these purposes – it acts both as a shielding stream and a constricting stream, offering both the improved efficiency and the higher rate of “blow away” effect.
The overall process of gas being turned into plasma is not exactly set in stone, either – it is also possible to replace gas with air and generate plasma from that, using the air plasma method. It does require a special type of electrode (zirconium or hafnium, combined with copper holder) that is harder and more costly to obtain, so it is not as simple as replacing expensive gas with regular air.
There are several other methods of plasma cutting that exist, too, such as the High Tolerance Plasma Arc Cutting method that was created to try and bridge the massive gap in cut quality between laser cutting and all of the other methods, but it does have its own limitations in terms of material thickness and overall performance in terms of cutting speed, so it’s not used as often.
All in all, plasma cutting is a rather popular cutting method, even though it is rather complicated – but it’s also used to create an abundance of appliances and metal details, so the market for plasma cutting is highly competitive.