During the process of plasma cutting, while piercing you may face some problems such as double arcing, plasma gas snuffing and bridging. These issues usually arise when the molten metal, which is at a temperature of around 1500°C, is directly blown back at the torch during the process of piercing.
Normally, the plasma arc is focussed and controlled by the swirling gases as it flows from the electrode to the plate via the nozzle. The layer of gas prevents any contact between the plasma arc and copper nozzle. In case the plasma arc touches the nozzle, it will cut the nozzle like any other conductive metal.
When any arc occurs that is secondary to the main arc during plasma cutting, it is known as double arcing. This usually occurs when the electric current passes through the copper nozzle or via any other conductive path onto the workpiece rather than through the arc column directly. This happens due to high amperage, if the plasma gas flow is very low, or due to some plasma jet problem. Double arcing usually occurs if the piercing occurs very close to the workpiece. The spray of the metal being cut hampers the plasma jet by interrupting the electric field around the arc thereby causing the arc to spread. This results in a nick, gouge or flowering of the nozzle orifice if the plasma arc is forced into the nozzle’s sidewall. This damages the nozzle, which may lead to a drop in the cut quality – like excess dross, inability to pierce the material, greater bevel angle, etc.
A more extreme kind of double arcing is known as bridging. During the process of plasma cutting, if a puddle of molten material is formed that touches the plate and the torch, then bridging occurs. A circuit is formed with the workpiece since the puddle of molten material is conductive electrically. The plasma arc flows to the plate through the lower resistance path which damages the nozzle and causes the shield also to fail. Today, even the most sophisticated copper shields that are electrically isolated are not immune to this kind of failure.
The most extreme kind of double arcing is called snuffing. When the nozzle of the torch or the shield is forced against the plate when the piercing begins, plasma gas snuffing occurs. For example, when the automatic height control of the torch pushes the plate and locates the initial height, and if the plate is very thin or bowed and is pushed down, the torch will not be set at the correct height. This usually happens during underwater plasma cutting where the person operating the cutter is unable to see the front of the torch. There is uncontrolled double arcing of the plasma due to snuffing. This results in the failure of the shield, electrode and the nozzle of the plasma cutter.
These problems occur very commonly during the process of plasma cutting. Often, the problems of double arcing, bridging and snuffing can lead to a drop in the cut quality and failure of the plasma cutter’s consumables. Hence, you must be very careful during the process of piercing or cutting.