Plasma Arc Cutting, also referred to as plasma cutting, is a processing method that uses the heat of a high-temperature plasma arc to locally melt (and evaporate) the metal at the cut of the workpiece, and uses the momentum of high-speed plasma to remove the molten metal to form the cut.
Figure 1 Plasma Arc Cutting
Brief of Plasma Arc Cutting
The plasma arc cutting process is illustrated in Figure 2. The basic principle is that the arc formed between the electrode and the workpiece is constricted by a fine bore, copper nozzle. This increases the temperature and velocity of the plasma emanating from the nozzle. The temperature of the plasma is in excess of 20,000℃ and the velocity can approach the speed of sound. When used for cutting, the plasma gas flow is increased so that the deeply penetrating plasma jet cuts through the material and molten material is removed.
Figure 2 Plasma Arc Cutting Process
The process differs from the oxy-fuel process in that the plasma process operates by using the arc to melt the metal whereas in the oxy-fuel process, the oxygen oxidises the metal and the heat from the exothermic reaction melts the metal. Thus, unlike the oxy-fuel process, the plasma process can be applied to cutting metals which form refractory oxides such as stainless steel, aluminium, cast iron and nonferrous alloys.
The power source required for the plasma arc process must have a drooping characteristic and a high voltage. Although the operating voltage to sustain the plasma is typically 50 to 60V, the open circuit voltage needed to initiate the arc can be up to 400V DC.
Process of Plasma Arc Cutting
On initiation, the pilot arc is formed within the body of the torch between the electrode and the nozzle. For cutting, the arc must be transferred to the workpiece in the so-called “transferred” arc mode. The electrode has a negative polarity and the workpiece a positive polarity so that the majority of the arc energy (approximately two thirds) is used for cutting.
In the conventional system using a tungsten electrode, the plasma is inert, formed using either argon, argon-H2 or nitrogen. However, oxidising gases, such as air or oxygen, can be used but the electrode must be copper with hafnium.
The plasma gas flow is critical and must be set according to the current level and the nozzle bore diameter. If the gas flow is too low for the current level, or the current level too high for the nozzle bore diameter, the arc will break down forming two arcs in series, electrode to nozzle and nozzle to workpiece. The effect of “double arcing” is usually catastrophic with the nozzle melting.
The quality of the plasma cut edge is similar to that achieved with the oxy-fuel process. However, as the plasma process cuts by melting, a characteristic feature is the greater degree of melting towards the top of the metal resulting in top edge rounding, poor edge squareness or a bevel on the cut edge. As these limitations are associated with the degree of constriction of the arc, several torch designs are available to improve arc constriction to produce more uniform heating at the top and bottom of the cut.
Figure 3 Plasma Arc Operation
Applications
Plasma arc cutting can increase the speed and efficiency of both sheet and plate metal cutting operations. Manufacturers of transportation and agricultural equipment, heavy machinery, aircraft components, air handling equipment, and many other products have discovered its benefits.
Plasma cutters are used in place of traditional sawing , drilling, machining, punching, and cutting. The high-temperature plasma arc cuts through a wide variety of metals at high speeds. Although plasma cutting can cut most metals at thicknesses of up to 4 to 6 inches , it provides the greatest economical advantages, speed, and quality on carbon steels under 1 inch thick, and on aluminum and stainless steels under 3 inches thick.
Plasma arc cutting has gained approval in both hand-held and automated cutting operations. Some of the most impressive results are achieved in automated systems. Advances in computer numerical controls (CNC) , robots, and other automation techniques have offered manufacturers higher cutting speeds achieved through plasma cutting. Improved torch designs and more efficient power supplies have made plasma cutting increasingly popular.
New areas of technology in plasma cutting systems include non-transferred arc plasma, which allows plastics and other nonconductive materials to be cut. Research on cutting plastics is continuing and at least one commercial process is currently available.
Figure 4 Plasma Arc Cutting with water
Safety
1. A sink should be set under plasma cutting. During the cutting process, the cutting part should be cut underwater to avoid the harm of smoke to the human body.
2. Avoid direct visual inspection of the plasma arc during plasma cutting. Professional protective glasses and facial masks are required to avoid arc burns to eyes and skin.
3. In the process of plasma cutting, a large amount of toxic gases will be generated, and it is necessary to ventilate and wear a multi-layer filter dust mask.
4. In the plasma cutting process, you need to wear towels, gloves, foot sheaths and other protective equipment to prevent the skin from being burned by sparks.
5. The high frequency and electromagnetic radiation generated by the high-frequency oscillator during plasma cutting can cause damage to the body. Some long-term practitioners even have symptoms of infertility. Although the medical community and the industry are temporarily inconclusive, they still need to do a good job of protection.
Picture Source: How does Plasma Cutting Work
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