You can easily cut a thick plate using a high-density plasma arc. However, the trickiest part of plasma cutting is the initial pierce. Here are some basic strategies to help you overcome this task and allow the plasma arc to pierce through the metal workpiece as easily and effectively as possible.
Dross and Double Arcing During Plasma Cutting
During the process of plasma cutting, till the plasma arc penetrates the bottom of the metal plate, molten metal is produced that spatters around the pierce area. This is quite an expensive mess.
Also, sometimes the operator is faced with the problem of double arcing. Typically, the plasma arc travels from the electrode to the metal; however, in the case of double arcing, the plasma arc travels from the electrode to the workpiece passing via the nozzle. This can damage the torch parts. Double arcing is usually caused due to the build-up of spatter in the front-end of the plasma torch. Excess build-up of spatter destroys the consumables prematurely.
Although modern plasma cutting systems can pierce metal 2 inches thick or more, the difficulty also increases and metal spatter is still produced. You can avoid this by using effective pierce strategies.
Some Pierce Strategies
It is ideal to use edge-start while cutting. However, this is not always possible. If you cut from the workpiece’s edge to any part outside the diameter, it produces plenty of scrap. If you want to cut any part inside the diameter or cut holes in the workpiece, then there is no option but to pierce directly.
When you are beginning the pierce, all the energy of the plasma is concentrated on the surface, but the energy spreads through a wider area when you pierce deeper. As you pierce deeper, sides are formed at the pierce hole. So apart from the energy being directed in the downward direction, it is also focussed on the side of the pierce hole. This causes more metal to melt and the hole at the top is widened further.
The high rate of gas flow can help to force the metal away from the pierce hole. During the process of carbon steel cutting, the oxygen gas shield reacts with the metal and helps to accelerate the pierce but at the same time it also widens the hole. This is applicable if you are doing your plasma cutting above water. However, if you are cutting the carbon steel under water, the metal spatter will solidify more quickly and build up higher on top of the metal plate.
In the case of metals like stainless steel and aluminium, oxygen shielding is not effective and this means that the plasma torch must pierce the metal by melting and blowing away the molten metal.
Thus, fabricators use several different techniques to achieve a precise and efficient pierce through thick metal plates whether stainless steel, carbon steel, aluminium, etc. The entire objective is to minimize waste and increase the cost efficiency of the cutting operation.