Plasma cutting is a process that works by sending an electric arc through a gas that is being forced through a restricted opening. By forcing the gas through a nozzle, it travels at high speed and is able to cut through metal that is made molten by the high temperature of the plasma.
Plasma cutting can use a variety of gases to create the plasma and is available in a number of different forms. This makes it a very flexible and affordable process that can be used to create very precise cuts quickly in several types and thicknesses of materials. As a result, it is now widely employed for a variety of applications in a wide range of industries.
What is the Working Principle of a Plasma Cutter?
As the name implies, this type of device uses plasma to perform the actual cutting. Plasma is the fourth state of matter, with solid, liquid and gas being the most commonly recognised states.
Changes from one state to the next are caused by energy in the form of heat. This changes a solid to a liquid and then, at an even higher temperature, from liquid to gas. This is typified by ice being heated to form water and then to become steam.
When gases are subjected to very high temperatures, plasma is formed. This consists of charged particles that are ionised, along with neutral atoms and molecules, and carries a great amount of energy that is released when it comes into contact with the material being cut
How Does a CNC Plasma Cutter Work?
Plasma cutters are comprised of three basic types:
- Manual Plasma Cutting – Hand-held torch, fully guided by the operator. No mechanisation, no automation. Lowest precision, used mainly for rough cutting, repair work, or when tolerances aren’t critical.
- Mechanised Plasma Cutting – The torch is mounted on a machine (like a gantry or track system), but the cutting path can be guided manually by the operator or by a basic non-computerised control system. This improves consistency over manual hand-held cutting, but doesn’t offer the repeatability or complex shape cutting of CNC.
- CNC Plasma Cutting – A form of mechanised cutting, but with computer numerical control guiding the torch along programmed paths. This is where you get precision, repeatability, beveling capabilities, and integration with CAD/CAM software.
All types have the same basic components:
- A power supply that converts single-phase or three-phase AC voltage into DC current at between 200 and 400 volts. Direct current is necessary because a constant level of power is needed to maintain a steady plasma arc for a precise cut, although the power level can be varied depending on the material being cut and its thickness.
- An arc starting console (ASC) that produces AC power of around 5,000 volts at two megahertz. This creates the spark inside the plasma torch to produce the plasma arc.
- A gas console, which controls the flow rate and pressure to ensure the ideal plasma arc.
- An air compressor or canister that supplies the gas, often compressed air, from which the plasma is created.
- A plasma torch, into which are funnelled the gas and electricity for the process. Each torch has a number of elements that need replacing over time — an electrode, swirl ring, nozzle, retaining cap and shield cap. As these elements wear, the plasma jet can be spoiled by impurities in the gas or inconsistent voltage, and so regular maintenance is essential.
All these components operate together for plasma cutting to work effectively. When the process is started, the power supply sends a DC current through the electrode and nozzle, at the same time starting the flow of gas. The gas is forced through the nozzle at high pressure and is heated by an electric arc to an extremely high temperature (up to 22,000⁰C) so that it forms plasma.
The plasma is so hot that it causes the metal in its path to become molten, cutting through it. The combination of high-velocity plasma and compressed gas then blows the molten metal away so that a clean cut results. The bigger the machine and the more power it can supply, the greater the thickness of cut that is possible.
What Kind of Gas Do You Use with a Plasma Cutter?
Plasma cutting can work with a variety of gases, and the ones you use will depend on the method of cutting and the type and thickness of the material to be cut.
- Air is the most economical gas to use and is generally delivered from an air compressor. With its composition of nitrogen and oxygen, it is entirely suitable for plasma cutting and is one of the fastest gases to use when cutting low-carbon steel. It can also be used with nitrogen or nitrogen and hydrogen to produce burr-free cuts in aluminium up to 6mm thick. However, when used on its own, air causes cut oxidation and slag hanging, and reduces efficiency due to the short service life that results for the nozzle and electrode.
- Argon is an inert gas that produces a stable plasma arc, so it hardly reacts with metals at high temperatures. Nozzles and electrodes have a longer service life than with other gases, but argon is rarely used on its own due to slag problems caused by the high surface tension of the molten metal. It is, however, often used with nitrogen or a combination of nitrogen and hydrogen to cut high-alloy steel or aluminium.
- Nitrogen produces a higher energy jet and more stable plasma arc than oxygen, particularly if a higher voltage supply is used. It can be used on its own or in combination with other gases, enabling the cutting of carbon steel at high speed and forming minimal slag, even when cutting high viscosity materials such as stainless steel or nickel-based alloys.
- Oxygen, when using a high temperature for the gas and high-energy plasma arc cutting, cuts low-carbon steel at high speed. It works best when paired with electrodes that are resistant to oxidation and high temperatures.
- Hydrogen is rarely used on its own but is often combined with other gases, particularly argon. This combination greatly increases the enthalpy, arc voltage and cutting ability of the plasma jet and produces one of the most powerful gases used for plasma cutting. Compressing with a water jet increases the cutting efficiency even further.
Whatever type of gas is used, it has to be as pure as possible for a fast and clean cut. Filters can be used to remove oil, moisture and particles, and so ensure an efficient process.
What Should You Not Cut with a Plasma Cutter?
The process heats gas to an extremely high temperature — typically up to 13,800°C (around 25,000°F) — ionising it and making it electrically conductive. This means it is capable of cutting any conductive material and is typically used on:
- Aluminium, where the process is more cost-effective than other methods for metal up to 160 mm thick.
- Mild steel that is commonly used due to its relatively low cost, high impact strength and ductility.
- Stainless steel, which is resistant to corrosion and up to 30 mm cutting thicknesses are possible.
- Brass is highly conductive but produces fumes during cutting that are harmful to health, so good ventilation is needed.
- Copper also requires good ventilation but has high conductivity and corrosion resistance.
- Cast iron is low-cost, malleable, has a low melting temperature and high compression strength, and is very conductive.
Any materials that are not conductive are not suitable for plasma cutting. This typically includes softer materials such as wood, plastic, ceramics and metals such as tungsten or lead, where other forms of cutting are needed.
What are the Disadvantages of Plasma Cutting?
Plasma cutting has many benefits that include its ability to cut conductive materials, including non-ferrous metals, quickly and with great precision. It can handle quite thick materials, is generally safer than traditional cutting methods that use grinding and sawing techniques, and can even be used underwater.
Like most processes, however, plasma cutting does have some downsides:
- It cannot be used on non-conductive materials.
- Thin materials can be cut too quickly so that cut edges are jagged or inconsistent and need finishing. In these cases, laser cutting may be more suitable.
- Very thick materials sometimes cannot be cut in one pass, and mechanical or water jet cutting may be better.
- It is a process that uses high voltage and heat, so there are safety issues that need to be addressed.
- UV radiation is created, and so eye and skin protection are needed.
- The process creates fumes and noise, which are counteracted by the use of a water table or ventilation system.
For the right materials and situations, plasma cutting can be unbeatable. It offers great flexibility for any machine or fabrication shop, being able to cut a variety of metals with precision and speed. At Plazmax, we offer a wide range of plasma cutting and other types of systems that can be custom-made, so we can provide one that exactly meets your needs. Browse our cutting products and get in touch if you need help or advice.