The cutting quality of plasma metal cutter machine is of great significance to the production of enterprises. In many fields where sheet metal plasma cutter is currently applied, the improvement of cutting accuracy and bevel slope will bring huge benefits to the secondary processing of workpieces. Here are five important factors that affect the quality of plasma cutting in detail.
The working gas and flow of CNC plasma cutting machine are one of the main parameters that affect the cutting quality. Air plasma cutting is currently one of many working gases. It is widely used due to its relatively low cost. The processing effect is indeed inadequate. The working gas here referred to includes the cutting gas and auxiliary gas. Some equipment also requires arcing gas. Generally, the appropriate one is selected according to the type, thickness and cutting method of the cutting material. The cutting gas must not only ensure the formation of plasma jet, but also ensure the removal of molten metal and oxides from the incision. Excessive gas flow will take away more arc heat, making the length of the jet shorter and resulting in reduced cutting capacity and unstable arc. Too small gas flow will result in the lost of straightness of the plasma arc sand makes the cutting depth shallow, and it is also easy to produce dross. Therefore, the gas flow must well match with the cutting current and speed. Currently most plasma arc cutting machines rely on gas pressure to control the flow, because when the torch diameter is constant, the gas pressure is also controlled to control the flow. The gas pressure used to cut plate of a certain thickness is usually selected according to the data provided by customer. If other special applications required, the gas pressure needs to be determined through actual cutting tests.
The most commonly used working gases include air, oxygen, nitrogen, argon, and H35, argon-nitrogen mixed gas.
1).Air contains about 78% nitrogen by volume, so the slag formed by cutting with air is very similar to cutting with nitrogen; air also contains about 21% oxygen. Because of the presence of oxygen, cutting speed of low carbon steel with air is also very high; air is also the most economical working gas. However, when air cutting is used alone, problems such as dross hanging, notch oxidation, nitrogen addition, etc., and low life of the electrodes and nozzles will affect the work efficiency and cutting cost.
2). Oxygen can increase the cutting speed of mild steel. When oxygen ts taken for cutting, the cutting mode is very similar to flame cutting. The high-temperature and high-energy plasma arc makes the cutting speed faster, but it must be used in conjunction with an electrode that resists high-temperature oxidation, at the same time, the electrode is protected against impact during arcing to prolong the life of electrode.
3).Hydrogen is usually used as auxiliary gas to mix with other gases. For example, the well-known H35 (volume fraction of hydrogen is 35%, and the rest is argon) is one of the gases with the best plasma arc cutting ability. As hydrogen can significantly increase the arc voltage, the hydrogen plasma jet has a high enthalpy value. When mixed with argon, the cutting ability of its plasma jet is greatly improved. Generally, argon and hydrogen are commonly used as cutting gases for metal materials with the thickness of 70mm or more. If water jet is used to further compress the argon + hydrogen plasma arc, higher cutting efficiency can also be obtained.
4).Nitrogen is a common working gas. Under the condition of higher power voltage, nitrogen plasma arc has better stability and higher jet energy than argon, even for cutting materials with high viscosity of liquid metal (Such as stainless steel, nickel-based alloy), the slag amount at the lower edge of cut is also very small. Nitrogen can be used alone or mixed with other gases. For example, nitrogen or air is often used as the working gas in automatic cutting. These two kinds of gases have become the standard gases for high-speed cutting of carbon steel. Nitrogen is sometimes used as an arcing gas in oxygen plasma arc cutting.
5). Argon hardly reacts with metal at high temperatures. The argon plasma arc is very stable, and the nozzles and electrodes used have a high service life. However, the plasma voltage of argon plasma arc is low, the enthalpy is not high and the cutting ability is limited. Compared with air cutting, the cutting thickness will be reduced by about 25%. In addition, in the argon-protected environment, the surface tension of the molten metal is large, which is about 30% higher than that in the nitrogen environment, so more problems of slag hanging occur. Even cutting with a mixture of argon and other gases has a tendency to stick to slag. Therefore, pure argon alone is rarely used for plasma cutting.
In addition to the influence of working gas on the cutting quality, the cutting speed also has an important influence on the cutting quality of CNC plasma cutting machine. The optimal cutting speed range can be selected according to the equipment description or determined by experiments. Due to the influence of factors such as thickness, material, melting point, thermal conductivity and the surface tension after melting, the corresponding change of cutting speed also occurs, which is main shown as below:
1).A moderate increase in cutting speed can improve the quality of the incision, that is, the incision is slightly narrowed, the surface of the incision is more flat and deformation can be reduced.
2).when the cutting speed is too fast, the linear energy of the cutting is lower than that of the required value. The jet in the slit cannot blow away the molten cutting melt immediately to form a large back drag. Surface quality is thus degraded.
3).When the cutting speed is too low, as the cutting place is the anode of plasma arc, in order to maintain the stability of the arc itself, the anode spot or the anode area must find a place where the current is conducted near the slit closest to the arc, at the same time, it will jet more heat in the radial direction of jet flow. Therefore, the incision will be widened, and the molten material on both sides of the incision will gather and solidify at the bottom to form hanging slags which is not easy to clean, and the upper edge of the incision will be rounded due to the excessive heating and melting.
4). When the cutting speed is extremely low, the arc may even extinguish as the cut is too wide.
It can be seen that the cutting quality is inseparable with the cutting speed.
3.The cutting current
The cutting current of CNC sheet metal plasma cutter machine is an important cutting parameter, which directly determines the cutting thickness and speed. The proper use of CNC plasma machines for high-quality fast cutting requires a deep understanding and mastery of the cutting parameters.
1).the arc energy and cutting ability increase as the increase of cutting current, also, the cutting speed increases accordingly.
2).Diameter of the arc increases as the increase of the cutting current, the incision becomes wider owing to the thicker of plasma arc.
3).Excessive cutting current increases the thermal load of the nozzle which damages he nozzle prematurely, the cutting quality thus declines even normal cutting cannot be performed.
When selecting the power source for plasma cutting, power source which is too large or too small is not the right option. It is a waste in cutting costs if the power source is too big, as a large amount of current cannot be used at all. When selecting the small plasma source in order to save the cutting cost, However, the cutting requirements cannot be met during actual cutting, which is a great harm to the CNC cutting machine itself!
Therefore, the cutting current and the corresponding nozzle should be properly selected according to the thickness of the material.
4.the nozzle height
The nozzle height of CNC plasma cutting machine refers to the distance between the tip of nozzle and the cutting surface, which forms one part of the entire arc length. Ad the plasma arc cutting generally uses the constant current or steep drop power source, the current changes are small after the nozzle height is increased, but it will increase the arc length and the arc voltage, which will increase the arc power. The arc length exposed to the environment increases, and the energy lost by the arc column increases.
Under the combined effect of the two factors, the former’s role is often completely offset by the latter, but it will reduce the effective cutting energy and cause the cutting ability to decrease. Generally, the blowing force of the cutting jet is weakened, the slag remaining in the lower part of the cut is increased, and the upper edge is over-melted to cause rounded corners. In addition, considering the shape of the plasma jet, the diameter of the jet expands outward after leaving the cutting torch, and an increase in the height of the nozzle necessarily causes an increase in the width of the cut. Therefore, it is beneficial to increase the cutting speed and cutting quality by adjusting the nozzle height as low as possible, but the double arc phenomenon may be caused when the nozzle height is too low. The ceramic outer nozzle can be used to set the nozzle height to zero, that is, the tip of the nozzle directly contacts the surface to be cut, thus the good effect can be obtained.
5.The Arc power
In order to obtain a high-compressibility plasma cutting arc, the CNC plasma cutting machine takes nozzles which have smaller hole diameter and longer channel length, and whose cooling effect has been enhanced. This can increase the current passing through the effective section of the nozzle, in other words, the power density of the arc increases, at the same time, however, compression also increases the power loss of the arc. Therefore, the actual effective energy used for plasma metal cutter machine is smaller than the power output by the power supply, and its loss rate is generally between 25% and 50%. Some methods, such as water compression plasma arc cutting, have greater energy loss rates. This should be considered when designing the cutting parameter or economics accounting of the cutting cost.
The thickness of metal plates used in the industry is mostly below 50mm. Within this thickness range, conventional plasma arc cutting often forms upper-large and lower-small cuts, and the upper edge of the cut will also decrease the cutting accuracy and increase the amount of subsequent processing works. When cutting carbon steel, aluminum and stainless steel with oxygen and nitrogen plasma arc, if the plate thickness lies within the the range of 10 – 25mm, usually the thicker the material is, the better the verticality of the end edge will be, and the angle error of its cutting edge is 1 Degree to 4 degrees. When the thickness of the plate is less than 1mm, as the thickness of plate decreases, the angle error of the cut increases from 3 °- 4 ° to 15 °-25 °.
It is generally believed that the cause of this phenomenon is due to the imbalance of the heat input of the plasma jet on the cutting surface, that is, the energy of the released plasma arc is more in the upper part than in the lower part. The imbalance of energy release is closely related to many process parameters, such as the degree of plasma arc compression, cutting speed, and the distance from the nozzle to the workpiece. Increasing the compression degree of the arc can prolong the high-temperature plasma jet and form a more uniform high-temperature region. At the same time, increasing the speed of the jet can reduce the width difference between the upper and lower cuts. However, the excessive compression of conventional nozzles often causes double arcs. Double arcs will not only consume electrodes and nozzles, making the cutting process impossible, but also cause a reduction in the quality of the incision. In addition, excessive cutting speed and excessive nozzle height for sheet metal plasma cutter will cause an increase in the width difference between the upper and lower cuts.