Plasma Arc Cutting (PAC)

Plasma Arc Cutting is a metal cutting process that uses a high temperature stream of ionised gas through a water-cooled nozzle at very high velocity. An arc is formed between the electrode and the workpiece, which is constricted by a fine bore copper nozzle. Oxygen oxidises the workpiece material, and it is melted by the exothermic reaction. The melted metal is then blown away from the line of cut. Temperatures can reach up to 20,000˚C. Manual (portable) or automated systems are common.
The plasma gases include argon, hydrogen, nitrogen and mixtures, plus air and oxygen.
PAC has a higher cutting speed and produces a smaller HAZ than oxy-fuel cutting

Typical Uses:

• Engine components.
• Sheet-metal fabrication.
• Domestic appliances.
• Instrumentation devices.
• Pipes.

Examples of uses

a. Typical functions for plasma-arc cutting.
b. Example of the process during operation.

Design guidelines for Plasma Arc Cutting

• Cutting, slotting and profiling operations are common.
• The process causes a heat-affected zone (HAZ).
• As the plasma process cuts by melting, a characteristic feature is the greater degree of melting towards the top of the metal resulting in top edge rounding, poor edge squareness, or a bevel on the cut edge.
• Material thickness up to 50mm can be cut, but with heavy-duty equipment, materials up to 150mm thick can be cut.

Process variations

• PAW (Plasma-arc welding). The difference between this and cutting is that a filler rod is used. The plasma provides the melting/fusion of the base materials and rod.
• PAS (Plasma-arc spraying). Solid feedstock is melted and sprayed onto substrate to alter surface properties (e.g. wear resistance/oxidation protection).
• High Tolerance Plasma Arc cutting (HTPAC) systems are competitive with laser cutting systems because they alloy better accuracy and a smaller HAZ than conventional PAC. (HTPAC can cut metals from 0.8 to 10mm thick).
• PAG (Plasma-arc Gouging) can create blind features, typically 12mm wide and 5 to 6mm deep.
• Water shielded PAC.

Tradenames/alternative names

• HTPAC (High Tolerance Plasma Arc Cutting).
• PBM (Plasma Beam Machining).

The environment

• Produces high noise levels.
• The heat potentially causes toxic fumes. Smoke and fumes can be reduced using the water-shielded process variation.
• PAC can be used underwater.

The economics

• Typically automated.
• This is a viable alternative to TIG welding for automation. HTPAC is a viable alternative to laser cutting.
• High equipment costs. Low finishing costs. Moderate labour costs.Economical for prototyping or low production runs.
• Filters and cooling water deionisers create addition maintenance and production costs.
• The electrode and nozzle may need replacement every 2 to 8 hours depending on processing conditions.

Technical notes

• The equipment has two modes of operation:

1. Melt-in fusion for reduced distortion (lower currents). This method is for welding.
2. Key-hole fusion for complete penetration of material thickness (higher currents). This method is for cutting.

• The choice of gas and mixture proportions is important depending on required operating conditions.
• The operating voltage to sustain the plasma is typically 50 to 60V. The open circuit voltage needed to initiate the arc can be up to 400V DC.
• Shielding gas is argon, argon-H2 or nitrogen for the method with the tungsten electrode.