The Plasma Torch Guide

What Is Plasma?

Image source: ResearchGate.net

“Plasma” has different meanings in medical and industrial contexts. The industrial context being relevant to plasma torches, it is what we will discuss here.

When the atoms of a gas are subjected to high energy, such as a strong electromagnetic field, electrons can cut loose from the atoms and move around freely. This ‘soup‘ of electron-deficient atoms (aka ions) and free electrons is called plasma.

Plasma is often described as the ‘fourth state of matter‘, subsequent to the solid, liquid and gas states.

What is a Plasma Torch?

A plasma torch is a tool for cutting or welding metals like steel, stainless steel, aluminum, brass or copper. It is typically used in metal fabrication shops, automotive repair and restoration shops, the salvage industry and scrapyards.

Overview of Plasma Torch Operation

Working diagram of a DC plasma torch

First a plasma torch creates a high-energy electric field  in a electrically neutral gas stream (flowing at 15l/mt) through its cylindrical housing. This causes electrons to gain the electric field’s energy and separate from the gas atoms, leaving a positively charged nucleus (this is called an ‘ion‘) and a ‘sea’ of free electrons (this process is called ‘ionization‘) . The ions and electrons all carry significant energy with them, derived from the electric field.

The ions and electrons then collide with other neutral atoms and ionize them, often releasing their carried energy in the process with the emission of intense heat (up to 40,000 deg F) and light.  

The pressurized plasma stream (called a plasma jet) exits through a nozzle onto the workpiece at high velocity.

The workpiece is always electrically grounded and therefore is itself a path for the plasma jet to go to ground. As the workpiece conducts the hot jet, the workpiece drastically rises in temperature. This heat can be used to either cut or weld the workpiece.

Some specifics:

• The gas is usually shop air, nitrogen, argon or oxygen
• The electric field can be generated from alternating current (AC), direct current (DC) or radio frequency current (RF).

Cutting With a Plasma Torch

Image source: Open University

Cutting metal is a popular use of the plasma torch because

• The cutting takes place at a high speed
• The cut is precise.

The plasma is blown out of the nozzle at high velocity onto the workpiece and the workpiece begins to melt. The high-velocity gas stream also blows the melted metal out of the way, which creates a deepening groove, finally resulting in a cut.

Plasma torches are ideally suited for cutting everything from thin sheet metal  0.6 inches thick to steel plate six inches thick.

The sharpness of the plasma jet enables the cutting of minute details on the workpiece. Take note that various operating parameters of the torch need to be configured to achieve that precision on a case-by-case basis. Example parameters are the operating voltage, the cutting air/gas pressure and the composition of the gas.

Dr. Robert Gage of Union Carbide’s Linde Division patented plasma cutting  on September 10, 1957 in the US.

Plasma Cutting System Components

Image source: teandersen.com

The above diagram depicts a simple plasma torch cutting system.

The power source (also known as the power supply) provides in sequence the various voltages the torch needs at settable current, and has manual controls to set the arc current. It also provides the cutting gas (air in the above case) to the torch

Welding With a Plasma Torch

1. Gas plasma, 2. Nozzle protection, 3. Shield Gas, 4. Electrode, 5. Nozzle constriction, 6. Electric arc

As in the case of plasma torch cutting, a  gas is converted to plasma within the torch and streamed through a narrow copper nozzle. This constriction increases the velocity of the plasma jet due to the Bernoulli effect to almost the speed of sound.  The jet hits the grounded workpieces at the desired welding spot and the intense heat melts the workpieces to form a weld.

As depicted in the above diagram, a second gas is forced all around the plasma jet to shield the weld from being oxidized by the surrounding air. The shielding gas is typically argon or argon plus 2 to 5% hydrogen; the plasma gas is usually argon.

The main advantage of plasma welding is the increased control over the arc, resulting in welds that have well-defined edges and smooth surfaces.

The same Dr. Robert Gage of Union Carbide’s Linde Division who patented plasma cutting patented plasma welding in 1953.

Plasma Torch Starting

Image source: thomasnet.com

Starting the plasma torch is achieved in three stages. 

1. The electrode is charged to a negative potential between 240 and 400V DC
2. Pressurized cutting gas is pumped through the torch and begins to swirl out of the nozzle as a result of flowing through the swirl chamber
3. The nozzle is automatically connected to the positive potential of the power supply
4. The electrode generates a short burst of high frequency, high voltage potential (about 15,000 volts, 2MHz), which causes a spark to jump between the electrode and nozzle. This spark ionizes the gas to some extent creating a small conductive path from electrode to nozzle
5. A DC arc (the ‘pilot arc’) now forms between the electrode and the nozzle. This of course consumes the nozzle during every use, which is why the nozzle must be replaced regularly
6. The gas flow forces the arc just produced onto the workpiece. The power supply disconnects the nozzle from the electrode and increases the amperage of the arc flowing onto the workpiece, creating the larger cutting arc (“main arc“)
7. Cutting or welding can now proceed.

Plasma Torch Maintenance

With proper preventive maintenance, a plasma torch will:

1. Utilize electrical power efficiently and thereby avert unnecessary increases in one’s power bill
2. Not make uneven cuts or welds because of excessive wear on parts
3. Reduce to near-zero the possibility of your torch failing in the middle of an important job.

Plasma Torch  Preventive Maintenance Procedure List

1. Clean the body of the torch. Disassemble the torch and check if the various threads are distinct. Clean the inside of the torch body with a liquid cleaner meant for electrical devices and wipe it dry with a clean swab. Unscrew the tube covering the leads and ensure the terminations are secure. Check for damages. Use a blower to remove any metal dust that may have crept into the area.
2. Wipe the torch leads down. Wipe them down to remove any dust or dirt on them. You could also use a blower if the contaminants are not stuck to the leads. Check for holes and kinks, bare or frayed wires. Use a meter to check if the shielding is grounded.
3. Check coolant-related components.  Check that the flow rate and pressure of the coolant liquid are as per the manufacturer’s specifications. Ensure that the mechanical switches in the coolant line function properly. Inspect the coolant line filters and pump screens for clogging. Clean them or replace them if they cannot be satisfactorily cleaned. Check coolant resistivity: it should be less than 10 microOhms or greater than 10,000 Ohms/cm resistance. Change the coolant every six months.