what is plasma cutting and how does it work

plasma cutting machine is a kind of processing method which uses the heat of high-temperature plasma arc to make the metal melt (and evaporate) locally at the cut of workpiece, and removes the molten metal by high-speed plasma momentum to form the cut.

Plasma cutting with different working gases can cut all kinds of oxygen cutting difficult metal, especially for non-ferrous metals (stainless steel, aluminum, copper, titanium, nickel) cutting effect is better; its main advantage is that the plasma cutting speed is fast, especially in cutting ordinary carbon when the thickness of the metal is small. When the steel sheet is thin, the cutting speed can reach 5-6 times that of the oxygen cutting method, the cutting surface is smooth, the thermal deformation is small, and there is hardly any heat affected zone.

Up to now, the working gas (the working gas is the conductive medium of the plasma arc, and also the heat carrier, but also to exclude the molten metal in the incision) has obvious influence on the cutting characteristics, cutting quality and cutting speed of the plasma arc. The commonly used plasma cutting working gases are argon, hydrogen, nitrogen, oxygen, air, steam and some mixed gases.

CNC plasma cutting machine is widely used in automobiles, locomotives, pressure vessels, chemical machinery, nuclear industry, general machinery, construction machinery, steel structure and other industries!

The choice of process parameters of NC plasma cutting machine is very important to the cutting quality, cutting speed and efficiency. The correct use of numerical control plasma cutting machine for high-quality rapid cutting, cutting process parameters must be deeply understood and mastered.

First, cutting current: It is the most important cutting process parameters, directly determines the cutting thickness and speed, that is, cutting ability. The influence is as follows: 1. The cutting current increases, the arc energy increases, the cutting ability increases, the cutting speed increases; 2. The cutting current increases, the arc diameter increases, the arc coarsens and the incision widens; 3. The cutting current too much increases the thermal load of the nozzle, the nozzle prematurely damages, and the cutting quality naturally decreases. It can't even cut normally. So before cutting, the cutting current and corresponding nozzles should be selected according to the thickness of the material.

2. Cutting speed: The best cutting speed range can be selected according to the equipment instructions or determined by experiments. Because of the thickness of materials, different materials, high and low melting point, thermal conductivity and surface tension after melting and other factors, the cutting speed also changes accordingly. Main manifestations: 1. Increasing the cutting speed moderately can improve the quality of the incision, that is, the incision is slightly narrower, the surface of the incision is smoother, and the deformation can be reduced. 2. The cutting speed is too fast to make the cutting line energy lower than the required value. The jet in the cutting slot can not blow the melt off immediately and form a large drag. With the incision slag hanging, the quality of the incision surface decreases. 3. When the cutting speed is too low, because the cutting place is the anode of the plasma arc, in order to maintain the stability of the arc itself, the anode spot or the anode area must find the place of conduction current near the slit nearest to the arc, at the same time, it will transfer more heat to the radial direction of the jet, so that the incision widens and the two sides of the incision melt. The material gathers at the bottom edge and solidifies, forming a hanging slag which is difficult to clean, and the upper edge of the incision is rounded by excessive heating and melting. 4, when the speed is very low, the arc will even die out because of the wide incision. It can be seen that good cutting quality is inseparable from cutting speed.

Three, arc voltage: it is generally believed that the normal output voltage of the power supply is the cutting voltage. Plasma arc cutting machine usually has a higher no-load voltage and working voltage, when using high ionization energy gas such as nitrogen, hydrogen or air, the voltage needed to stabilize plasma arc will be higher. When the current is constant, the increase of voltage means the increase of the arc enthalpy and the improvement of cutting ability. If the enthalpy is increased at the same time, reduce the diameter of the jet and increase the gas flow rate, often can get faster cutting speed and better cutting quality.

Fourth, working gas and flow: CNC plasma cutting machine working gas including cutting gas and auxiliary gas, some equipment also requires arcing gas, usually according to the type of cutting materials, thickness and cutting methods to choose the appropriate working gas. The cutting gas should not only ensure the formation of plasma jet, but also ensure the removal of molten metal and oxide in the incision. Excessive gas flow will take away more arc heat, shorten the length of the jet, lead to the decline of cutting ability and arc instability; too small gas flow will make the plasma arc lose its due straightness and make the cutting depth shallow, but also easy to produce hanging slag; so gas flow must be with cutting current and speed Good coordination. The current plasma arc cutting machine mostly relies on gas pressure to control the flow, because when the gun aperture is fixed, the control of gas pressure also controls the flow. The gas pressure used by numerical control plasma cutting machine to cut certain thickness material is usually selected according to the data provided by the equipment manufacturer. If there are other special applications, the gas pressure needs to be determined by actual cutting test. The most commonly used work gases are argon, nitrogen, oxygen, air, and H35, argon nitrogen mixed gases.

1. Argon almost does not react with any metal at high temperature. Argon plasma arc is very stable. Moreover, the nozzle and electrode used have high service life. But argon plasma arc has low voltage, low enthalpy and limited cutting ability. Compared with air cutting, its cutting thickness will be reduced by about 25%. In addition, the surface tension of the molten metal in the argon protection environment is larger, about 30% higher than that in the nitrogen environment, so there will be more slag hanging problem. Even if the mixture of argon and other gases is used, there will be a tendency to stick slag. Therefore, plasma argon cutting is rarely used alone.

2. Hydrogen is usually used as an auxiliary gas to mix with other gases, such as the well-known gas H35 (35% hydrogen volume fraction, the rest is argon) is one of the strongest plasma arc cutting gas, which is mainly beneficial to hydrogen. Hydrogen can significantly increase the arc voltage and hydrogen.

Plasma jet has a high enthalpy value, when mixed with argon, the cutting ability of plasma jet is greatly improved. Generally, argon and hydrogen are used as cutting gases for metal materials with thickness of 70mm or more. If the water jet is used for further compression of argon + hydrogen plasma arc, a higher cutting efficiency can also be obtained.

3. Nitrogen is a commonly used working gas. Under the condition of higher power supply voltage, nitrogen plasma arc has better stability and higher jet energy than argon. Even when cutting liquid metal with high viscosity such as stainless steel and nickel-based alloy, the slag hanging on the lower edge of the incision is very small. Nitrogen can be used alone or mixed with other gases, such as nitrogen or air, which are often used as working gases in automatic cutting, and have become standard gases for high-speed cutting of carbon steel. Sometimes nitrogen is also used as arcing gas when oxygen plasma arc cutting.

4. Oxygen can improve the speed of cutting low carbon steel. When using oxygen for cutting, cutting mode and flame cutting is very imaginary, high-temperature and high-energy plasma arc makes cutting faster, but must cooperate with the use of high-temperature oxidation-resistant electrode, while the electrode arc protection against impact to extend the life of the electrode.

5. The air contains about 78% of nitrogen, so it is very imaginary to use the air to cut the slag formed by hanging with nitrogen; the air also contains about 21% of oxygen, because of the presence of oxygen, the cutting speed of low-carbon steel materials with air is also high; at the same time, air is the most economical working gas. However, when air cutting is used alone, there will be problems such as slag hanging, incision oxidation and nitrogen increase, and the lower life of electrode and nozzle will also affect the efficiency and cutting cost.

5. Nozzle height: refers to the distance between the nozzle end face and the cutting surface, which forms part of the whole arc length. Because NC plasma cutting machine generally uses constant current or steep drop external characteristic power supply, the change of current is very small when the nozzle height increases, but it will increase the arc length and lead to the increase of arc voltage, so as to improve the arc power; but it will also increase the arc length exposed to the environment, and increase the loss of arc column energy. Under the combined action of two factors, the former is often completely offset by the latter, but the effective cutting energy will be reduced, resulting in a reduction in cutting ability. Usually manifested in the cutting jet blowing force is weakened, the incision bottom residual slag increases, the upper edge of the melt and appear rounded corner. In addition, considering the shape of the plasma jet, the diameter of the jet expands outward after leaving the muzzle, and the increase of the nozzle height will inevitably lead to the increase of the notch width. Therefore, choosing the smallest nozzle height is beneficial to improve cutting speed and cutting quality. However, if the nozzle height is too low, it may cause double arc phenomenon. The ceramic nozzle can be used to set the nozzle height to zero, that is, the nozzle end face directly contacts the cutting surface, and good results can be achieved.

6. Cutting power density: In order to obtain high-pressure plasma arc cutting, cutting nozzles are used smaller nozzle aperture, longer hole length and strengthen the cooling effect, so that the nozzle effective section through the current increased, that is, the power density of the arc increased. But at the same time, compression also makes the power loss of the arc increase. Therefore, the actual effective energy used for cutting is less than the power output, and the loss rate is generally between 25% and 50%. Some methods such as water compression plasma arc cutting energy loss rate will be greater, in the cutting process parameter design or cutting cost economy. Accounting should consider this problem.

For example, the thickness of metal plate used in industry is mostly less than 50 mm. In this thickness range, conventional plasma arc cutting will often form a large and small cutting edge, and the upper edge of the cutting edge will lead to the decline of the accuracy of the cutting size and increase the follow-up processing. When using oxygen and nitrogen plasma arc cutting carbon steel, aluminum and stainless steel, when the plate thickness is in the range of 10-25 mm, usually the thicker the material, the better the verticality of the end edge, its cutting edge angle error is 1-4 degrees. When the plate thickness is less than 1mm, the cutting angle error increases from 3 degrees ~4 to 15 degrees ~25 when the thickness of the plate is reduced.

It is generally believed that this phenomenon is due to the unbalanced heat input of plasma jet on the cutting surface, that is, the energy release of plasma arc at the upper part of the cutting surface is more than that at the lower part. This imbalance in energy release is closely related to many process parameters, such as plasma arc compression, cutting speed and nozzle.