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Plasma cutters work by sending an electric arc through a gas that is passing through a constricted opening. The gas can be shop air, nitrogen, argon, oxygen. etc. As electricity from the cutter torch travels down this plasma it delivers sufficient heat to melt through the workpiece. As the metal being cut is part of the circuit, the electrical conductivity of the plasma causes the arc to transfer to the work.
In the following paragraphs, we have tried to simplify complex phenomena such as spark arcs, torch, high, cut quality, plasma arc, plasma gas and plasma work to levels that you can quickly go through before starting out on the state of matter of your first cutting session.
Phase I: The Pressurized Air/Gas and the Electrode
While most of the plasma reviews we’ve dealt with focus on the nozzle and the electrode, what these represent are mere enclosures for the real scene of developments. This scene comprises of a narrow shaft that runs through the body of the torch, the plasma gas at and through the nozzle (and the electrode that surrounds the mouth).
Connected to the source of pressurized gas, this shaft is narrow enough to ensure that the plasma gas that passes through it during operation does not spread out and thus lose its pressure.
Surrounding the nozzle, of course, is the electrode. Made of base metal and endowed with a hafnium tip for improved use and conductivity, it sits around the nozzle’s mouth. By default, this electrode has a negative potential (similar to the negative end of a battery holder) and thus is primed to transfer electricity from the torch to the metal.
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Phase II: Circuit and Plasma
When one of the best plasma cutters is turned on, it begins to force pressurized air through the mouth. Pressurized air by itself, however, does not produce either the heat required for the plasma to work. As we’d mentioned elsewhere, this heat is produced instead by one of two methods:
- Contact method: The metal and the nozzle physically touch, creating a circuit that allows the electricity from the electrode to discharge itself . This creates a spark/arc that sets off the cutting procedure.
- High Frequency Method (using pilot arc): Another method of setting off a spark is to use a high frequency unit that creates a pilot arc without requiring the unit to be in actual contact with the machine. The HF cut quality torch reaches towards the metal and once it comes into contact with it, it makes it high thus transforming it into the arc.
In either case, the plasma arc that is created produces and makes use of a region of extreme heat at the mouth of the nozzle. This heat transforms the gas passing through the nozzle into plasma. To make understanding how does a plasma works easier, let us mention at this point that the name itself derive from this plasma, which is the fourth state of matter where superheated matter (atoms, ions, etc) loses its earlier characteristics (eg. the features of a gas) and becomes a mass of particles. Plasma is a good conductor of electricity and ensures that the circuit is maintained.
In case of the plasma work, this plasma reaches a temperature of about 35,000 degrees Fahrenheit. Further, since the air/gas is pressurized, the plasma retains the directional nature of the gas as well as the speed, thereby creating a fast moving, accurately aimed mass of plasma.
Phase III: Cutting and The “shielding gas”
Once the plasma hits the steel, it passes the heat onto the machine itself. If you’re wondering how does a plasma works in such a situation since metals are good conductors of heat, let us add that the extreme temperature and fast plasma speed ensure that the transfer of heat is not fast enough. This leads to the equipment being in the immediate vicinity of the oncoming plasma to melt into liquid slag.
Such slag would remain in that state if it is held in position, but quite inevitably, the machine is moved to new areas. In doing so, it“cuts” through the liquid slag, thus producing a “plasma cut” around which the slag solidifies into steel once more once it has moved on. All of this occurs in milliseconds, giving the impression that the machine is cutting through the tool like a saw.
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The process of plasma can also be completed in two ways – either the plasma cutters move right through the tool and thus causes the nozzle to lose touch with it, or the electricity is switched off. In most cases, the former method is followed as it allows the user to make precise cuts resulting in plasma arc.
Notably, once the equipment has lost contact with the machine, it will also lose the spark/arc and cannot recreate a pilot arc (if it is an HF pilot arc unit) as this requires a special starter mechanism. It would hence have to be started many times to handle complex cuts. On the plus side, it ensures that there are no injuries associated with excess running time.
All of this, however, does not account for the benefits of using plasma cutters– the presence of extremely narrow slag belts/kerf areas along the cut. Slag belts/kerf areas are parts of the tool surrounding the cut that contains oxidized deposits of molten slag.
Such deposits can arise if the arc is erratic or too diffused, thereby heating up nearby impurities and allowing for the reaction of such impurities with oxygen (from the air) to produce unsightly marks on the equipment.
To solve this, many machines come with a shielding gas mechanism. A shielding gas blows around the cutting gas and is often non-reactive. Such a non-reactive shield allows the plasma to stay focused and prevents the oxygen in the surroundings from interacting with the molten slag.
While some of the cutters are small and simple enough for people to observe how plasma cutters works in detail, others will provide only limited insights.
Whatever the case, one should be rest assured that most of these equipments work along the lines given above and should any part malfunction, knowledge of the above can help locate the truant parts and replace them in the shortest possible time.
Further, knowledge of the procedure can help avoid injuries arising out of the improper handling of parts. This is especially so since the melting and solidifying of the machine is too fast and most humans consider the process as akin to proper “cut”.
While delving into the physics of the plasma cutter may not be necessary for the person who wishes to answer the question – how to use and maintain a plasma by taking it to a costly maintenance shop, the person who truly wants to have full control over the functioning and maintenance of these, needs to have some idea of how does a plasma works.
In light of these benefits of knowing the procedure, we sincerely hope that this article will make plasma cutting a safer and easier operation.