Key features of plasma cutting

Plasma is defined as a gas in which a certain portion of the particles are ionised. The presence of a large number of charge carriers makes the plasma electrically conductive so that it responds strongly to electromagnetic fields. Plasma, therefore, has properties quite unlike those of solids, liquids, or gases and is considered to be a distinct state of matter.

For arc cutting, we take advantage of the fact that plasma is electrically conductive. Being so hot, not even its own atoms can remain unaffected and they get ionised, that is they emit free electrons which can then freely move from one atom to the other and the atoms themselves are left with a positive charge. Positevely and negatively charged particles give plasma its electro-conductive property. What do we have here? We have an excessively hot gas mass than can also conduct electricity. The idea of forming a jet of this gassy mass to attack and melt metals was only one step ahead.

For this idea to be realised, some kind of a “plasma gun” should be produced. It should be enough heat resistant to contain the plasma and designed in a way to be able to target it at the metal to be cut. Because no material would remain intact at 22000 degrees Celcius, there was only one way out: Design a “gun” in such a way that the ionised gas be “enveloped” in a much cooler gassy “container”, the parts involved provided with constant cooling and make sure they be made of a material of the highest possible melting point. Additionally, it should constrict the plasma jet so that its thermal energy would concentrate on a small area of the target material, for better results.

A plasma torch uses a copper alloy nozzle to constrict the ionised gas stream to focus the energy to a small cross section.

The high velocity gas jet ejected through the nozzle transfers electric current to the plate we wish to cut which is melted and the molten material is driven away by the very plasma jet. In our drawing to the right please see a Plasma torch design with or without Swirl gas, about which we explain right underneath: A. Coolant entry, B. Coolant exit, C. Plasma Gas, D. Swirl Gas, E. Cutting direction and F. Cut surface.

Swirl gas

The introduction of Swirl gas technology assists cutting in many ways. To begin with, gas swirling helps cooling. The non-ionised atoms of the gas are heavier and cooler than the ionised ones and, when forced to swirl, are distributed on the outer layer of the swirling gas “column”. This lower temperature layer protects the copper alloy nozzle. The higher the current, the greater the percentage of ionised atoms, so that the “ideal ratio” of 30% plasma, 70% cool gas gets higher, increasing the eating of the nozzle and reducing its cooling. Nozzles are designed and manufactured to function within a given range of current amperage.

Swirl gas improves cut quality. If plasma jet is not swirling, both kerf sides would be bevelled, sometimes to an extent that makes work-pieces useless (please cheque drawing to the right: A. Straight gas flow, cut surface bevelled on both sides). By swirling the gas, the plasma jet is distibuted on one side evenly, therefore this cut surface is “square” (Figure to the right B. Swirled gas jet, square cut surface). If swirling direction is changed (clockwise instead of anti-clockwise), the square side changes diametrically. The swirling gas jet attacks one side of the sheet to be cut vertically in its full thickness. Cutting energy is distributed evenly over the full thickness of the work-sheet, resulting in a cut surface perpendicular to the one of the work-sheet. This is the cut edge to use; the one across the kerf is inclined at an angle of some 5 to 8 degrees.

Shield gas introduction can further constrict the jSaldos - Entrega gratuita