ESAB Knowledge center.
The Basics of Plasma Cutting
May 6, 2014
Plasma Cutting Process
- Plasma cutting is the fastest cutting process on carbon steel, aluminum, or stainless steel.
- Plasma cutting can be combined with waterjet or oxy fuel on the same part.
- Plasma cutting can be use for precision cutting on gauge material up to 6” thick stainless. ESAB invented plasma cutting in 1955, and we've never stopped developing ways to make plasma cutters better and easier. ESAB's latest innovations in the plasma cutting process automation increase production with more consistent cut quality every time ... regardless of operator skill level:
Characteristics Plasma is defined in Websters as "collection of charged particles ... containing about equal numbers of positive ions and electrons and exhibiting some properties of a gas but differing from a gas in being a good conductor of electricity ..."
For arc cutting, plasma can also be defined as an electrically heated gas stream. The gas stream is heated to such high temperature that it becomes ionized. The ionized gas by definition can then freely exchange electrons between atoms. This electron movement is what allows the gas to carry the cutting amperage.
A plasma torch uses an alloy copper nozzle to constrict the ionized gas stream to focus the energy to a small cross section. The principle is the same as using a magnifying glass to concentrate the sun's energy to create intense heat.
The gas flowing through the nozzle also serves as a medium to remove the molten metal heated by the ionized gas. Approximately 30% of gas is actually ionized (under optimum conditions) while the remaining 70% of the gas stream is used for material removal and cooling.
Swirling the gas assists cutting in several ways. Swirling increases cooling. The un-ionized gas atoms are heavier/cooler and are thrown to the outside of the spinning gas stream. This cool barrier provides protection for the copper nozzle. As amperage is increased, the amount of ionization increases (changing the 30/70% ratio) and cooling decreases, shortening the life of the nozzle. Nozzles are designed to operate within a specific current (amp) range.
Swirling gas improves cut quality.
If the plasma gas is not swirled, results would be a bevel on both sides of the cut. By swirling the gas, the arc is distributed evenly along one side of the cut. If the swirl is reversed direction (CW to CCW), the square side will switch. As the ionized gas (plasma arc) is swirled, the electrical arc will attach itself evenly to the leading edge of the cut. These multiple attachment points provide a more even power distribution through the workpiece. This equalizing of power from top to bottom results in a squarer side. The other side having a 5 to 8 degree bevel.
The introduction of a shield gas will further constrict and cool the nozzle. This gas is injected in the plasma stream after the ionization process at the tip of the nozzle.
Water injection improves cut quality and cools the nozzle. By swirling water in the same direction as the gas, then injecting it at the point where the arc exits the nozzle, the arc is further constricted. When cool water comes in contact with the high temperature arc, a steam layer between the arc and nozzle bore is formed. The effects of this barrier can be demonstrated by heating a frying pan and pouring water on it. Immediately small beads of water will dance on the pan surface instead of vaporizing. These water beads are protected by the steam insulating properties formed when the water comes in contact with the pan. Water temperature must stay below 70 degrees F. for water injection to work correctly. A condition known as film boiling occurs if the temperature rises above that point. An unstable arc, shorter nozzle life, and poor cut quality will result.
Starting a Plasma Arc
There are three major components inside the torch body.
- Gas Baffle (Swirl Baffle)
These items are called consumables. They are consumed over time during the plasma process and must be replaced. Parts from the ESAB PT-36 torch are shown above. Other torches may appear different but the all have parts that function as the 3 major listed above. Consult your torch manual for the exact part configuration.
The electrode is connected to the negative side of a DC plasma power supply. The nozzle is connected to the positive side but is electrically isolated by means of a normally open relay.
The following occurs when a start input is given to the plasma system:
- The main contactor within the power supply energizes placing a high negative voltage on the electrode.
- Gas begins to flow to the torch and is swirled by the baffle.
- The normally open contacts in the nozzle circuit close providing a path to the positive side of the power supply.
- A high frequency generator provides a high frequency-high voltage potential between the electrode and nozzle. This causes a small spark to jump between nozzle and electrode, ionizing a path through the gas.
- Along this ionized path, a larger DC arc begins to flow between the electrode and nozzle. This is called the pilot arc.
- The pilot arc is blown out of the nozzle by the gas flow, and contacts the work piece.
- The main arc is created when the pilot arc transfers to the work material (if the torch is close enough). The nozzle relay opens removing the nozzle from the circuit. A trans- ferred arc condition has been established.
- The main arc increases to cutting amperage after the nozzle relay is opened.
A double arc is a condition which allows the nozzle to stay in the plasma circuit. As described above, the nozzle should only be in the circuit during the pilot arc phase. If left in the circuit, the nozzle will carry cutting amperage which will destroy it.
Double arcing is caused by:
- Standing pierce. The torch has to be positioned close enough to the work-piece to allow the pilot arc to contact the plate, so the main arc can transfer. Pierce spatter is ejected at a shallow angle during the initial pierce. As the arc penetrates the material the spatter becomes more vertical. This debris may connect the plate and nozzle, keeping the nozzle in the circuit even when the relay opens to remove it. This scenario may damage the front end of the torch.
- Torch in contact with the plate. Cutting thin material. All automatic torch positioning systems utilize some initial height sensing method to position the torch above the plate. One method is the touch and retract method. The torch travels until it makes contact with the plate and retracts to the initial start height utilizing a timer or encoder. If the touch is not sensed properly, the torch may still be in contact with the material due to springing up or material warping. The nozzle will remain in the plasma circuit carrying cutting amperage, damaging it.
- Pilot arc malfunction. This can occur if the pilot arc relay circuit fails to remove the nozzle. This can happen either with a shorted relay or resistor. Again the nozzle is left to carry more current than intended, damaging it.
Preventing the Double Arc
Double arcing usually occurs during the piercing sequence.
Some techniques which can help avoid double arcing are:
- Creep move. The cutting machine is programmed at a reduced speed to begin machine movement on arc transfer. This speed is usually 5 to 10% of normal cutting speed and is for a given time period. Pierce spatter is being ejected away from the nozzle during this time. This reduces double arcing possibility.
- Torch rising during standing pierce. On arc transfer the torch begins to pull away from the work- piece. This allows the pierce spatter to clear the nozzle. This retraction continues for a timed period, and then lowers to correct cutting height after the machine is moving at cutting speed.
- Higher than normal initial height pierce (standing pierce). This allows the pierce spatter to miss the nozzle reducing the chances for a double arc. This method of prevention is the least effective.
Plasma Process Variables
The variables involved in plasma cutting must all be closely controlled to achieve maximum cut quality, maximum nozzle/electrode life and maximum production. A balance must be maintained between them.
The purity of gas is essential for good cut quality and long electrode life. Minimum purity requirements for nitrogen at 99.995% and 99.5 % for oxygen. If purity levels are less than recommended minimum the following could occur.
- Inability of the arc to penetrate thin materials at any current level.
- Depending on the degree contamination, variation in cut quality.
- Extreme short electrode life.
- When cutting with N2, appearance of a black film residue on the face of the electrode and in the nozzle bore. The worse the contamination, the more the residue. If the gas is pure, the electrode and nozzle bore will take on a sand blasted appearance.
Each nozzle is designed to perform at an optimum current based on a given gas pressure/flow. Increasing this pressure can result in a decrease in electrode life. This is evident by a drilled appearance in the tungsten insert. With nitrogen there will be a problem with torch starting. If the torch fails to start at high pressure, a sputtering pilot arc may be observed. Where high gas pressure may create problems, low gas flow will usually result in a double arc failure.
The Water Injection plasma process requires de-ionized and filtered water. Suspended solids, dissolved minerals and other factors affect the conductivity of water and nozzle life and increase the possibility of high frequency interference.
Cut Water Pressure/Flows
Cut water flow rate should be set to the amount specified in your torch literature. Excessive water flow will result in short electrode life and an unstable arc. Low water flow will result in insufficient cooling affecting nozzle life.
Kerf is the width of material (perpendicular to the torch and cut axis) removed during the plasma cutting process. Kerf is affected by three major variables.
- Cutting Speed. Faster cutting speeds with other variables constant will result in a narrower kerf. The kerf will continue to narrow until loss of penetration occurs. Slower travel speeds will result in a wider kerf until loss of arc occurs.
- Cutting Amperage. Increasing cutting amperage with the other two variables constant will result in a wider kerf. Continuing to increase current will widen kerf until the nozzle is destroyed. Lowering amperage will result in a narrower kerf, a more positive cut angle until penetration is lost.
- Standoff. Standoff is the distance maintained between torch and work-piece after piercing (while cutting). Most modern systems use an arc voltage feedback system. Increasing the arc voltage increases the standoff distance and widen the kerf. Continuing to increase standoff will eventually lead to loss of cut. Lowering standoff will lead to a narrower kerf and even- tually loss of cut.
Arc voltage is not a independent variable.
It is dependent on:
- Current (amperage)
- Nozzle orifice size
- Cut gas flow rate
- Cut water flow rate (if applicable)
- Cutting speed
Gases required for most applications are a start gas, shield gas, and cut gas. A few situations require a second shield gas. Results vary with different combinations of nitrogen, oxygen, air, methane, and H-35 (a combination of 35% hydrogen - 65% argon). Argon gas is used for plasma marking. Material type and thickness, cutting quality, speed, and production cost are variables to consider when selecting gas combinations. All gases are not appropriate for some applications and torches. Consult your torch literature for more information.
The ESAB Solution
ESAB's m3 plasma system fully automates the plasma cutting process, making it easy to set up all of the process variables discussed above, and achieve consistent cut quality.