Best Gases for Plasma Cutter

The Plasma Cutter which was developed in the 1960s is used to cut any type of conductive metal like cast-iron, steel (stainless-steel, mild-steel& carbon-steel), aluminium, nickel (and alloys), magnesium, copper (and alloys) and brass.

How are gases used in a Plasma Cutter?​

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Gases are essential to how a Plasma Cutter works. The gas within the Plasma Cutter is heated (up to 30,000OF) till it ionises, i.e. the gas separates into a collection of positive ions and electrons; which means it is now able to conduct electricity.

The ionised gas passes over an electrode which generates an electric arc which is conducted through the ionised gas (plasma).

The gas used for this purpose is called the primary gas or plasma gas.

Then this plasma gas is pressurised and send through a very narrow nozzle so as to focus the energy (both the heat and the electrical energy) on to a narrow cross section of the metal for a smoother, more precise and faster cut.

The heated gas melts the metal and the pressure of the gas removes the molten metal from the area of the cut. About 30% of the gas used is usually ionised, the rest 70% is used to remove the material and for material removing and cooling (or shielding) process.

There are also dual flow plasma cutters which use a secondary gas to act as a blanket or a shield to protect the material.

These gases are also called swirl gases. Some plasma cutters also use water as the shielding and cooling medium.

For more details on the process of plasma cutting:

How do you select the best gas for your plasma cutter?

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​There are various types of gas combinations that are used as the primary and secondary gases in the plasma cutter. Traditionally the older versions of plasma cutters have used Nitrogen as the primary or plasma gas and Carbon Dioxide as the secondary gas; however the more modern plasma cutters use shop air (combination of Nitrogen and Oxygen at 80% / 70% and 20% respectively) as the primary and secondary gas.

There are also other versions which use Oxygen, Methane and H-35 (combination of Hydrogen and Argon at 35% and 65% respectively).

The properties of gases which impact the cutting process are ionisation and dissociation energy, thermal conductivity, both of which impact the heat of the gas and thus speed of cutting and atomic weight (ease of removing molten material).

Let us look at properties of the various gases / gas combinations used in plasma cutting:​

1. AIR

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​The most versatile and the cheapest of all the gases in the list as this need not be purchased, the air in the shop can be used with the help of a dedicated air compressor, a refrigerated dryer and a series of filters to remove any contaminants (such as particulate, oil mist and moisture). Shield mediums are air and water.

Recommended for:

  • Mild Steel
  • Stainless Steel
  • Aluminium
  • Versatile
  • Cheap
  • Minimal dross
  • Cut surface can get oxidised and need to be cleaned.
  • Issues with welding the cut edges.

2. Nitrogen

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This is the original gas used for plasma cutting. It has a medium atomic mass and conductivity. Shield mediums recommended are air, Carbon Dioxide, and water.

Recommended for:​

  • Stainless Steel
  • Aluminium under 1/2”
  • Cut quality
  • Life of parts
  • Smooth shiny cut surface
  • No oxidation
  • Rounded top edge
  • Black cut surface.

3. Oxygen

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Gas with medium atomic mass and conductivity. Fastest cutting gas especially for mild steel. Shield is air.

Recommended for:

  • Mild Steel
  • Fast cutting speed
  • Minimal dross
  • Cut surface can get oxidised and need to be cleaned.
  • Expensive.

4. H-35

Argon is an inert gas with high atomic mass which when combined with lightweight but highly conducting and easily ionisable hydrogen produces the hottest burning plasma. Shield gas recommended is Nitrogen.

Recommended for:​

  • Thick Stainless Steel(6”)
  • Thick Aluminium(6”)
  • Fast cutting speed
  • Minimal dross
  • Cut surface can get oxidised and need to be cleaned.
  • Expensive.

The decision of which gas is best for you depends on:

  • The type and thickness of material being cut.
  • The type of plasma cutter being used.
  • The cutting quality that you are looking for.
  • The cost of gas and production.
  • The speed of your plasma cutter.
  • Expensive.

The decision making process starts with you reading the literature given when you purchased the plasma cutter. This will give you the gases which are compatible with your plasma cutter. Then depending on the type and thickness of the material that is being cut.

The table below taken from Praxair, one of the world’s biggest suppliers of industrial gases gives you an easy reckoner for selecting the best gas for your work is:​

Plasma Gas

Shield Gas

Mild Steel

Stainless Steel

Aluminum

Air

Air

Good cut quality

Economical

Good cut quality

and speed Economical

Good cut quality

and speed Economical

Oxygen

Air

Excellent cut quality

and speed Very little

dross

Not recommended

Not recommended

Nitrogen

Carbon

Dioxide​

Fair cut quality Some

dross Excellent parts

life

Good cut quality

Excellent parts life​

Excellent cut quality

Excellent parts life​

Nitrogen

Air

Fair cut quality Some

dross Excellent parts

life

Good cut quality

Excellent parts life​​

Excellent cut quality

Excellent parts life​​

Nitrogen

Water

Fair cut quality Some

dross Excellent parts

life​

Excellent cut quality

Excellent parts life​

Excellent cut quality

Excellent parts life​

Argon

Hydrogen​

Nitrogen

Not recommended

Excellent on

thicknesses above

1/2"​

Excellent on

thicknesses above

1/2"​

(From: https://www.praxairdirect.com/Industrial-Gas-and-Welding-Information-Center/Welding-and-Fuel-Gases/gases-plasma-cutting.html)

Whatever gas is used, you need to pay attention to the purity of the gas, and you need the minimum purity of

  • The type and thickness of material being cut.
  • The type of plasma cutter being used.
  • The cutting quality that you are looking for.
  • The cost of gas and production.
  • The speed of your plasma cutter.

If the gas is less pure then the quality of the cut suffers as the ionisation process is imperfect due to the presence of contamination and the life electrode also can get reduced.

Select your gas carefully and smartly for the best use of your plasma cutter.

  • April 3, 2017
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