Conventional Plasma Arc Cutting

The plasma jet generated by conventional "dry" arc constriction techniques was introduced in 1957 by Union Carbide's Linde Division. In the same year, Dr. Robert Gage obtained a patent, which for 17 years gave Union Carbide a virtual monopoly. This technique could be used to sever any metal at relatively high cutting speeds. The thickness of a plate could range from thin sheet metal to plates as thick as ten inches (250 mm). The cut thickness was ultimately dependent on the current-carrying capacity of the torch and the physical properties of the metal. A heavy duty mechanized torch with a current capacity of 1000 amps could cut through 10-inch thick stainless steel and aluminium. However, in most industrial applications, plate thickness seldom exceeded two inches. In this thickness range, conventional plasma cuts were usually bevelled and had a rounded top edge. Bevelled cuts were a result of an imbalance in the heat input into the cut face. A positive cut angle resulted because the heat energy at the top of the cut dissipated as the arc progressed through the cut.

Plasma Arc Cutting

This heat imbalance was reduced by placing the torch as close as possible to the workpiece and applying the arc constriction principle, as shown in Figure 1. Increased arc constriction caused the temperature profile of the electric arc to become extended and more uniform. Correspondingly, the cut became more square. Unfortunately, the constriction of the conventional nozzle was limited by the tendency of increased constriction to develop two arcs in series, one arc between the electrode and nozzle and a second arc between the nozzle and workpiece.

This phenomenon was known as "double arcing" and damaged both the electrode and nozzle. Double arcing severely limited the extent to which plasma cut quality could be improved. Since the introduction of the plasma arc process in the mid-50's, considerable research has focused on increasing arc constriction without creating double arcing. Plasma arc cutting as performed then is now referred to as "conventional plasma cutting." It can be cumbersome to apply if the user is cutting a wide variety of metals and different plate thicknesses. For example, if the conventional plasma process is used to cut stainless steel, mild steel, and aluminium, it is necessary to use different gases and gas flows for optimum cut quality on all three metals.




Conventional plasma cutting predominated from 1957 to 1970, and often required very expensive gas mixtures of argon and hydrogen.