How does plasma cutting work? – And why isn’t there a puddle of copper there… Part 1

When talking to someone who is not in the metal fabrication industry and I tell them I work with plasma cutting equipment, I often get the question ‘What is plasma cutting?’  That’s the wrong question to ask me – unless you have some time on your hands….

First – a definition of plasma.  From dictionary.com : “A highly ionized gas containing an approximately equal number of positive ions and electrons”.  There are actually many different types of plasma gas (see Wikipedia)- we are interested in the high temperature type as the high temperature does the cutting in our application.

I get asked sometimes about the temperature of the arc – I am told it is in the neighborhood of 10,000 deg F to 30,000 deg F.  That’s some serious heat!!

Just in case you were wondering – the surface of the sun (a typical star) is about 7,800 – 10,000 degrees F.

In plasma cutting equipment we generate this plasma gas by electrical discharge – or more commonly refered to as an ‘arc’.  I’ll discuss this arc generation process in another blog.  After the plasma arc gets started, a large DC power supply pumps between 40 and 800A of current through the electrically conductive plasma gas.  This creates the super-heated gas – the plasma – that does the cutting of the material by quickly and accurately melting it away.

 

Now – if the plasma gas is at 10,000 degrees, what keeps the nozzle, shield and the electrode from vaporizing?  They are made mostly of copper, which vaporizes at about 4700 degrees.  The answer is the secret of plasma cutting – fluid dynamics.

You see just generating the plasma gas does nothing without focusing it in a small enough spot and with enough velocity to remove the material on the workpiece.  It is the fluid dynamics in the torch that both focuses (or constricts) the arc and provides thermal protection to the torch consumables.  The torch and consumable set is designed so that cooler shield gas is swirled around the hot plasma gas providing a buffer between the hot plasma gas and the relatively fragile copper consumables.  Here’s a cross-section view that can help:

goodshieldgoodshieldgoodshield

When the shield gas flow is correct it is constricting the plasma gas ‘flame’ to keep it contained and away from the nozzle orifice.

badshield

When the shield gas flow is insufficient the plasma gas ‘flame’ is much less constrained and will damage the nozzle and perhaps even the shield.

We’ll explore the gases used in the plasma and shield gas flows in the next post….