n this article, we’ll go through the most critical factors that contribute to the quality of plasma cutting.
The gas is used for the cutting process. The process may involve more than one gas, for example, a primary gas and a second gas. Currently, the air is widely used as a medium gas because of its relatively low cost. Some equipment also requires arc starting gas. The actual process that is selected for the work depends on the material and thickness of the workpiece and the cutting method that is used.
The medium gas is used to form the plasma jet and remove the molten metal and oxide generated in the cutting process. Excessive gas flow will take away more arc heat, making the length of the jet shorter, resulting in reduced cutting capacity and arc instability. A gas flow too small will cause the plasma arc to lose its straightness and cutting strength. It makes a shallower cut and is more likely to produce slag. Therefore, the gas flow must be compatible with the cutting current and speed. Plasma arc cutting machines mostly rely on gas pressure to control the flow rate because when the torch aperture is fixed, the gas pressure also controls the flow rate. The gas pressure used to cut a certain thickness of the material is usually selected according to the customer’s requirement specifications. For certain special applications, tests must be performed to determine the gas pressure. The most commonly used gases include argon, nitrogen, oxygen, air, H35, and argon-nitrogen mixed gas.
A: Air contains about 78% of nitrogen; in terms of volume, cutting with air generates a kind of slag that is very similar to cutting with nitrogen. Air also contains about 21% of oxygen. The presence of oxygen can make the cutting process faster. The cutting of low-carbon steel materials can also be performed at high speed. In addition, the air is a highly accessible resource with fewer costs. These facts make the air a widely adopted medium gas. However, using air alone for the cutting has downsides,. such as slag, cut oxidation, and nitrogen increase. Additionally, the reduced life of the electrode and nozzle can negatively impact productivity and bring up the cost.
B. Oxygen can increase the speed of cutting mild steel materials. In this sense, using oxygen for cutting is very similar to flame cutting. The high-temperature and high-energy plasma arc make the cutting process faster. However, to extend the electrode life, this process must be performed with an electrode that resists high-temperature oxidation, and that is protected against impact during arcing, .
C. Hydrogen is usually used as an auxiliary gas to be mixed with other gases. For example, the well-known gas H35, a mixture of 35% hydrogen and 65% argon, is one of the gases with a strong plasma arc cutting strength because of the presence of hydrogen. Hydrogen can significantly increase the arc voltage, so the hydrogen plasma jet has a high enthalpy value. When mixed with argon, its plasma jet cutting strength is greatly improved. Generally, for metal materials with a thickness of more than 70mm, argon + hydrogen is commonly used as the gas. If a water jet is used to compress the argon + hydrogen plasma arc further, a higher cutting efficiency can also be achieved.
D. Nitrogen is a commonly used gas. Powered with a higher voltage, nitrogen plasma arc has better stability and higher jet energy than argon, even when cutting liquid metal with high viscosity materials such as stainless steel. For cutting nickel-based alloys, the amount of dross occurring at the lower edge of the cut is also small. Nitrogen can be used alone or mixed with other gases. For example, nitrogen and air are often used as medium gases in automated cutting processes. These two gases have become the recommended options for high-speed cutting of carbon steel. Sometimes nitrogen is also used as the starting gas for oxygen plasma arc cutting.
E. Argon gas hardly reacts with any metal at high temperatures, and the argon plasma arc is very stable. Moreover, the nozzles and electrodes used have a long service life. However, the voltage of the argon plasma arc is low, the enthalpy value is not high, and the cutting strength is limited. Compared with air cutting, the thickness of the cut will be reduced by about 25%. In addition, in the argon gas protection environment, the surface tension of the molten metal is relatively large, which is about 30% higher than that in the nitrogen environment, so more slag will be generated. Even cutting with a mixture of argon and other gases has a chance of producing slag. Therefore, pure argon is rarely used alone for plasma cutting.
The cutting speed is also a major consideration when sourcing a plasma cutting machine. Each plasma cutting system comes with a designed speed range. Users can tune the speed according to the product instructions or by performing tests. In general, the speed can be determined based on factors such as the thickness, material, melting point, thermal conductivity, and the surface tension after melting the workpiece.
A moderate increase in cutting speed can improve the quality of the cut. It makes the cut slightly narrower and the cut surface smoother, reducing the chance of deformation.
If the cutting speed is too high, the linear energy of the cutting can be lower than the required energy. The jet in the slit cannot quickly blow away the melt immediately, so a large amount of trailing drag forms.
If the cutting speed is too low, overheating occurs. The anode of the plasma arc is where the cut actually occurs. Therefore, in order to maintain the stability of the arc itself, the CNC spot inevitably turns to a conduction current near the slit that is closest to the arc. In this way, the jet transmits more heat radially. In this case, the incision is widened. The molten material on both sides of the incision gathers and solidifies along the lower edge, forming a slag that is not easy to clean, and the upper edge of the incision is heated and melted to form a rounded corner.
When the speed is extremely low, the arc will even be extinguished due to the incision being too broad.
The current (amperage) determines the thickness and speed of the cutting. Therefore, the current is a key factor for performing high-quality rapid cutting. Specifically, the current affects these aspects:
- With a higher current, the system generates higher arc energy, a higher cutting strength, and a higher cutting speed.
- With a higher current, the system generates an arc with a larger diameter, producing a thicker cut.
- An excessive current, however, puts an abnormal thermal load on the nozzle. This gives the nozzle a shorter life and negatively impacts the cutting quality.
The power supply for your plasma cutting system must match the amperage planned for the cutting. A more-than-enough amperage incurs unnecessary costs. However, an amperage too small may not only negatively impact the cutting performance but also damage the cutting system.
The nozzle height refers to the distance between the nozzle end face and the workpiece, which is part of the entire arc length. Plasma arc cutting generally uses a constant current or steep drop external power supply.
Effects of a larger height:
When the nozzle height is increased, the amperage changes little. However, the increased arc length causes the arc voltage to increase and therefore causes the arc power to increase. At the same time, the longer arc translates to more exposure to the surroundings and thus more energy loss. This energy loss inevitably reduces the effective cutting energy, resulting in a reduction in cutting strength. In this case, because the blowing force of the cutting jet is weakened, you may find more residual slag at the lower edge of the incision, and the upper edge is over-melted to produce rounded corners. In addition, considering the shape of the plasma jet, the diameter of the jet expands outwards after leaving the torch mouth, and an increase in the height of the nozzle inevitably causes an increase in the width of the cut. Therefore, to improve the cutting speed and cutting quality, users usually select a nozzle height as small as possible.
Effects of a smaller height
However, when the nozzle height is too low, it may cause a double-arc phenomenon. Using a ceramic outer nozzle, you can set the nozzle height to zero; that is, the end face of the nozzle directly contacts the workpiece, and produce a high-quality cut.
To form a highly compressed plasma arc, the nozzle uses a smaller nozzle aperture and a longer hole length and strengthens the cooling effect. This can increase the current passing through the nozzle’s effective cross section so that the power density of the arc increases. However, the higher compression also increases the power loss of the arc. Therefore, the effective energy used for cutting is smaller than the power output by the power supply. The loss rate is generally between 25% and 50%. With certain methods, such as water compression plasma arc, the energy loss rate will be greater. You also need to consider this when designing your cutting process and planning your costs.
In most industrial applications, plasma cutting is used to cut metal plates with a thickness of less than 50mm. Cutting with conventional plasma arcs within this thickness range often results in deviations in cut sizes along the upper edge of the cut and therefore increases the amount of additional processing that is required. When using oxygen and nitrogen plasma arcs to cut carbon steel, aluminum, and stainless steel, if the thickness of the plate is in the range of 10 ~ 25mm, usually the thicker the material, the better the perpendicularity of the end edge. The angle tolerance of the cutting edge is 1-4 °. If the plate thickness is less than 1mm, as the plate thickness decreases, the incision angle deviation increases from 3 ° – 4 ° to 15 ° – 25 °.
It is generally believed that the energy of the plasma arc is released more to the upper part of the cut than to the lower part. This imbalance of energy release is closely related to many process parameters, such as the degree of plasma arc compression, cutting speed, and the distance between the nozzle and the workpiece. Increasing the compression of the arc can extend the high-temperature plasma jet to form a more uniform high-temperature area and, at the same time, increase the velocity of the jet, which can reduce the width difference between the upper and lower cuts. However, excessive compression of conventional nozzles often results in double arcing, which not only consumes electrodes and nozzles, making the process impossible but also leads to a decrease in the quality of the cut. In addition, excessively high speed and excessively high nozzle height will increase the difference between the upper and lower widths of the cut.
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