Plasma Cutting Process - How It Works
Plasma – The Fourth State of Matter
Matter exists in four states. They are solid, liquid, gas and plasma. Adding ever increasing energy causes a solid to become a liquid, then a gas. And finally, a plasma is created when enough energy is applied. The resulting plasma is now capable of carrying electrical current like a copper wire. Lightning is naturally occurring plasma.
Plasma Cutting Process
Plasma arc cutting is capable of cutting all metals. The process is commonly used to cut steel, stainless steel and aluminum. Plasma cutting systems range in amperage output from 20 to 1,000 amps. High definition plasma systems using oxygen as the plasma gas have a capacity of piercing and production cutting steel up to 2.0” at 400 amps. High power systems using plasma gas mixtures of argon and hydrogen have the capacity to cut up to 6.0” stainless or aluminum at 1,000 amps.
Plasma Arc Cutting (PAC) power supplies are the Constant Current (CC) type as are powers supplies used for stick electrode (SMAW) and TIG welding (GTAW). However, PAC power supplies output much higher voltage. Open circuit voltage may be as high as 400 VDC. Constricting the plasma jet through a nozzle increases the arc density. Plasma cutting torches include a component to swirl the plasma gas forcing non-ionized gases outward. Swirling the plasma gas causes the cooler un-ionized boundary layer of gas around the plasma column to become wider, squeezing the arc and further increasing the arc density, length and temperature. Swirling the plasma gas also increases nozzle life. The plasma arc column can achieve temperatures of approx. 24,000º K (or 42,000º F).
Pilot Arc (Non-Transferred)
A low current non-transferred pilot arc is established within the torch between the negatively charged electrode and the positively charged nozzle to provide a path for the cutting arc to transfer to the positively charged work piece. Plasma cutting systems with output over 125 amps ionize the plasma gas by applying a momentary burst of high frequency between the electrode and nozzle. The thousands of volts of high frequency provides a path for the DC component of the pilot arc. The instant DC current is measured between the electrode and nozzle, the high frequency component of the pilot arc is turned off. The plasma gas flow causes the DC arc to be blown out of the orifice and reattach to the face of the nozzle.
If the torch is close enough to the work-piece while the pilot arc is on, the main cutting current will seek to attach to the work piece. When current flow is measured between the electrode and the work piece, the DC component of the pilot arc is turned off. The high temperature, high velocity plasma stream melts and blows the molten material through the work-piece.
The plasma power supply supplies a contact closure to the cutting machine when the main cutting arc has transferred. This signal is often called the “OK to Move” signal. Cutting machine motion is delayed until the arc has fully penetrated to material. This delay is called “Pierce time”.
The cutting arc normally passes through the nozzle orifice without contacting the nozzle. A condition called a “double arc” occurs when the cutting arc attaches to the work piece via the nozzle. A double arc normally severely damages the nozzle. A single double arc can render the nozzle incapable of producing a quality cut.
A Double Arc May Occur When:
- The nozzle is too close to the material during the pierce
- Excessive metal builds up on the face of the torch shield
- Machine motion is initiated prior to the arc penetrating the metal causing molten metal to attach to the front of the torch
- Plasma gas flow is too low
- Cutting amperage is set too high
Basic Torch Design
Plasma torches are either a single gas or dual gas design. Typically, single gas design torches operate at up to 125 amps and are cooled by the gas flowing through them. The single gas input is split inside the torch into the plasma and shield flows. Over 125 amps, the increased heat of the arc requires that the torch is liquid cooled. Virtually all liquid cooled torches are the dual gas type. Dual gas torches have separate pathways for the plasma and shield gases allowing the torch leads to deliver the gases to the torch head through two separate hoses. The plasma and shield gases can therefore be different and delivery pressures and flows can be different as well.
Direction of Cut Path
Because the plasma gas is rotating, the cut is squarer on one side. With a clock-wise swirl direction, the cut on the side in the direction of travel is the squarer side. CNC controls designed for plasma cutting automatically produce motion that accounts for this directionality.
Systems with Single Gas Torches
Low cost automated plasma cutting systems are configured with single gas torches designed to cut all metal types using shop compressed air. Prices for automated air plasma cutting systems range between $3,500 (40 amp) and $10,000 (125 amp). Please note that these prices are for a plasma cutting power and torch – not a complete CNC cutting machine. This type of plasma system has become extremely popular with metal fabricators doing ornamental metal work and relatively low production general purpose plate cutting.
Systems with Dual Gas Torches
Today’s automated precision plasma cutting systems are priced at approximately $40,000 (130 amp systems), $75,000 (400 amp systems) and $125,000 (800 amp systems). Please note that these prices are for the plasma cutting power and torch – not a complete CNC cutting machine. They are configured with liquid cooled dual gas torches, computer-controlled switch-mode power supplies and sophisticated automatic gas delivery systems. Cut charts embedded into today’s CNC controls automatically adjust cutting parameters and select the required gases based on the material and thickness selected. Also, most precision plasma systems include technology which ramps amperage and gas flow at the start and stop of every cut. This technology significantly extends consumable life and delivers extremely consistent cutting performance over the life of consumables
Plasma Cutting Terminology
Plasma cutting does not produce cut edges that are exactly 90° to material surface. The bevel angle is amount the cut is off from square.
Non-conductive material molded within the torch body that electrically insulates between the negative and positive portions of the torch.
A relatively low-cost plasma cutting system comprised of a DC power supply, torch and a low-cost gas delivery system such as pressure regulators attached to the source of gases. Such systems produce arcs with a density of approximately 30,000 amps per square inch. Cut bevel angles of 5° to 7° are common. These systems may be offered for both hand and mechanized applications. When used on a CNC cutting machine in a steel application, the smallest recommendation hole is generally 2 times the material thickness, i.e. 1/2" in 1/4" material.
A transferred arc that passes through the nozzle and is established between the electrode insert and the material being cut.
Material that remains on the bottom of a plate after cutting. Dross can also be on the top of the plate, usually when cutting height is excessive.
|Dual Gas Torch||
Torches designed to operate with separate inputs for the plasma and shield. Dual gas torches may be either air cooled or liquid cooled. The plasma and shield gases may be the same or different. By optimizing the plasma gas and shield gas based on material and thickness, dual gas torches produce the highest cut quality.
Copper element with an insert of tungsten or hafnium. The electrode is connected to the negative output of the DC power supply.
|High Definition Plasma||
A significantly higher priced plasma cutting system comprised of a sophisticated switch-mode DC power supply, precision torch and a computer-controlled highly accurate gas delivery system mounted very near the torch. Such systems produce arcs with extremely high arc densities of 60,000 amps per square inch or more. Cut bevel angles of 1° to 3° are common. These systems are offered only for mechanized operation. When used on a CNC cutting machines in a steel application, the smallest recommended hole is 1 times the material thicknes, i.e. 1/4" in 1/4" material.
The amount of material removed by cutting arc. CNC controls offset the cut path by 1/2 the kerf to insure the final part is the correct size. Kerf widths range from .019" at 45 amps using N2/N2 cutting stainless steel to .340" at 400 amps using O2/Air cutting mild steel.
|Nozzle (tip, orifice)||
Copper element to focus the plasma stream. The nozzle is connected to the positive output of the DC power supply via a normally open relay contact. The pilot arc relay is momentarily closed to create the non-transferred pilot arc.
A non-transferred arc established between the electrode and nozzle in a plasma torch. The pilot arc excites the orifice gas (plasma) into becoming plasma. In liquid cooled torches, the arc is commonly comprised of a momentary burst of high frequency and followed by DC current. Many low amp air cooled air plasma torches use a moving element within the torch to "strike" an arc between the electrode and nozzle which eliminates the need for high frequency.
Orifice gas used to conduct current to and through the metal being cut.
|Shield Cap (Cup)||
Directs shield gas (secondary) flow around the nozzle (tip) and at the cut. In some torch designs, it also holds the parts in the torch.
|Shield Gas (Secondary)||
Cooling and cut assist gas. Torch design focuses flow around the nozzle to reduce the possibility of a "double arc", squeeze the cutting arc and to wash slag off the bottom of the cut.
|Single Gas Torch||
Torches designed to operate with a single supply of gas input, generally shop compressed air. Single gas torches split the input flow into plasma and shield flows within the torch. These torches are normally air cooled.
|Swirl Ring (Gas Distributor)||
Non-conductive material which includes extremely small holes that cause the plasma gas to swirl. Some torches use the swirl ring to set the gap between the electrode and nozzle.
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