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Selection of Process Parameters for Plasma Cutting

When plasma cutting, the input energy mainly depends on the arc power and cutting speed. The high arc power can obviously increase the cutting speed accordingly, thus obtaining high productivity. The improvement of arc power can be achieved by increasing the cutting current or working voltage, but the increase of current often accelerates the burning of electrodes and nozzles, so it is generally hoped to increase the arc power by increasing the arc voltage. But the arc voltage is not a parameter that can be adjusted completely independently, it is also affected by other parameters. Through the following analysis, the selection of the main process parameters and their influence on the cutting process are briefly introduced.

The main process parameters of plasma cutting include: type and flow of working gas, no-load voltage and working (cutting) voltage, nozzle aperture, electrode shrinkage, distance between nozzle and workpiece (nozzle height), working (cutting) current and cutting speed etc.

The type of cutting material and the thickness of the cutting piece are the basis for selecting the cutting process parameters. If the thickness of the material is large, a larger arc power and nozzle aperture should be selected. Workpieces with the same thickness but different materials have different cutting parameters. For example, pure copper and stainless steel, although the melting point of pure copper is lower than that of stainless steel, but its thermal conductivity is 18 times that of stainless steel, so when cutting pure copper, a larger arc power must be selected.

The influence and selection of the type of working gas on the plasma arc cutting process has been described in detail in plasma welding, and will not be repeated here.

The gas flow rate is generally determined according to the nozzle hole diameter and the thickness of the material. The gas flow rate is large, the compression degree of the arc is enhanced, the impulse of the plasma arc is also large, and the thickness that can be cut is large. However, if the flow rate is too large, the arc will be unstable, and the cold air flow will take away the heat of the arc too much, which will reduce the cutting ability and deteriorate the quality of the cutting edge. Table Effect of Nitrogen Flow on Cutting Quality shows the effect of nitrogen flow on cutting quality.

Effect of Nitrogen Flow on Cutting Quality
Effect of Nitrogen Flow on Cutting Quality

Usually, the working gas flow rate of a certain kind of cutting torch has been determined during the design, generally the gas flow rate can be supplied according to the specified value, and it should not be changed arbitrarily. When the thickness of the cutting material varies greatly, some adjustments can be made appropriately.

Plasma cutting——No-load voltage and cutting voltage

Cutting voltage is one of the most important process parameters in the cutting process, but it is not an independent process parameter. In addition to being related to the no-load voltage of the power supply, it also depends on the type and flow of the working gas, the structure of the nozzle, the nozzle and the The distance between workpieces and cutting speed, etc. After these parameters are determined, the cutting voltage is naturally determined. If the gas flow rate increases and the distance between the nozzle and the workpiece increases, the cutting voltage will increase accordingly. The no-load voltage is related to the ionization degree of the working gas used. According to the type of working gas and the cutting thickness, it has been determined during the design of the cutting power supply, but it will affect the cutting voltage.

Generally speaking, the higher the working voltage, the higher the arc power, and the higher the cutting ability. When cutting stainless steel with large thickness in China, the method of increasing the cutting voltage without increasing the cutting current is often used. However, when the voltage is high, especially when cutting by hand, there are safety problems.

Plasma cutting——Nozzle aperture

The nozzle aperture is determined according to the thickness of the cutting material and the type of working gas. When using argon or the mixture of argon and hydrogen, the nozzle aperture should be selected as a smaller value, and when nitrogen is used as the working gas, it should be selected as a larger value. For low current plasma arc cutting, because the thickness range of the material to be cut is small, usually the cutting torch is only equipped with a nozzle with one aperture.

PLasma cutting——Electrode shrinkage

Schematic diagram of electrode shrinkage ΔLy and nozzle height H

The shrinkage of the electrode refers to the distance ΔLy from the tip of the electrode to the inner surface of the nozzle (Fig. Schematic diagram of electrode shrinkage ΔLy and nozzle height H). Since ΔLy is not easy to measure, under the condition of known nozzle orifice length, it is often expressed as Ly.

The shrinkage of the electrode is a very important parameter, which greatly affects the arc compression effect and the burning loss of the electrode, thus greatly affecting the cutting effect and cutting stability. The larger the shrinkage, the better the compression effect of the arc, but if it is too large, the stability of the arc will be poor, and it is easy to produce double arcs and burn the nozzle. If the shrinkage is too small, the arc cannot be well compressed, and the electrodes are easily burned. If the electrode tip protrudes into the nozzle hole, the cutting ability will be reduced or even impossible to achieve cutting. The best position of the electrode tip should be in the suction area of  the air flow. In this case, the tip is in a relatively vacuum state, the electrode is not easy to be burned, and the arc can also be well compressed. In principle, it is generally appropriate to take 8~11 mm.

Plasma cutting——Distance between nozzle and workpiece

The distance H between the nozzle and the workpiece (Figure Schematic diagram of electrode shrinkage ΔLy and nozzle height H) has a significant impact on cutting efficiency and kerf width. If the distance is too large, the time for the arc to pass through the space is too long, the radiation heat loss will increase, and the arc column will spread, the cutting speed will inevitably decrease, and the incision will widen. If the distance is too small, although the cutting speed can be accelerated, it is easy to cause double arcs when cutting with high current. If it is too small, it may cause a short circuit between the nozzle and the workpiece. Usually, the distance between the nozzle and the workpiece should be reduced as much as possible under the condition that the nozzle and the workpiece are not short-circuited. For workpieces of general thickness, it is advisable to take 6~8 mm. When cutting workpieces with greater thickness, it can be increased to 10~15mm. The cutting torch should be perpendicular to the surface of the cutting workpiece. In order to facilitate the removal of slag, the cutting torch can also maintain a certain backward angle.

Plasma cutting current

The cutting current should be determined according to the size of the nozzle aperture. Figure Relationship between Nozzle Aperture, Cutting Current and Cutting Thickness-a shows the relationship between the cutting current and the nozzle aperture. The cutting current can also be selected according to the following formula:      

                    I= (70 ~ 100)d

                    I——Cutting current (A);

                   d——Nozzle aperture ( mm)。

Relationship between Nozzle Aperture, Cutting Current and Cutting Thickness
Relationship between Nozzle Aperture, Cutting Current and Cutting Thickness

For the nozzles that have been determined, there is a most effective cutting current value, at this time the limit cutting speed is the largest. If the cutting current is too large, the cutting speed will drop instead, and double arcs will easily occur. In addition, the thickness and material of the workpiece need to be considered when selecting the cutting current (Figure Relationship between Nozzle Aperture, Cutting Current and Cutting Thickness-b). Obviously, if the thickness of the workpiece is large, the current should also increase, but the nozzle aperture also needs to be increased accordingly. Different materials, such as cutting copper with equal thickness, because of the high thermal conductivity of copper, the cutting current should be increased.

In order to prevent severe burnout of nozzles, the allowable critical current values ​​are specified for nozzles of various apertures, see Table 3Limiting current to prevent severe nozzle burnout. See Table Adaptive working current of nozzles with different apertures during cutting for the applicable working current of nozzles with different apertures during cutting.

plasma cutting Parameters

Plasma cutting speed

Slag formation at the bottom of the incision

Plasma cutting speed is not only an important indicator reflecting cutting productivity, but also greatly affects cutting quality. When the cutting speed is high, the incision area is less heated, the incision is narrow, and the heat-affected zone is small. However, if the speed is too fast, slag will stick to the lower edge of the incision and even the cutting surface, and even the workpiece cannot be cut through. If the cutting speed is too slow, not only the cutting efficiency will decrease, but also the incision will widen, the inclination of the cutting surface will increase, and the slag will form a tumor at the bottom of the incision (Fig. Slag formation at the bottom of the incision), and the cutting quality will deteriorate. Table Effect of Cutting Speed ​​on Cutting Quality shows the influence of cutting speed on cutting quality under the condition that the cutting current and voltage are basically the same.

Effect of Cutting Speed ​​on Cutting Quality

Usually, the cutting speed is appropriate when there is no sticky dross or a small amount of dross hanging on the lower edge of the incision, even if there is a slight amount of drag. In addition, when cutting thin metal by hand, due to the limitation of manual speed, the cutting speed can generally only reach about 1m/min. Therefore, attention should be paid to selecting a cutting torch with a suitable working current according to the material and thickness of the workpiece in order to obtain good cutting quality. If a cutting torch with a higher power is selected, the cutting quality will deteriorate because the manual speed is lower than the appropriate cutting speed at this power.