Plasma is a gas heated to extremely high temperature and highly ionized.
It transfers the arc power to the workpiece.
The high heat melts and blows the workpiece away to form the working state of plasma arc cutting.
It is a processing method to remove the molten metal by the momentum of high-speed plasma to form a notch.
Let’s popularize the basic principle of plasma cutting machine.
Introduction to plasma cutting
Plasma cutting combined with different working gases can cut all kinds of metals that are difficult to cut with oxygen, especially for non-ferrous metals (stainless steel, aluminum, copper, titanium and nickel);
Its main advantage is that when cutting metal with small thickness, the plasma cutting speed is fast, especially when cutting ordinary carbon steel sheet, the speed can reach 5 ~ 6 times of oxygen cutting method, the cutting surface is smooth, the thermal deformation is small, and there is almost no heat affected zone!
Plasma cutting machine is widely used in automobile, locomotive, pressure vessel, chemical machinery, nuclear industry, general machinery, engineering machinery, steel structure and other industries.
Working principle and process of plasma cutting
1. After the compressed air enters the cutting torch, it is distributed by the gas chamber to form plasma gas and auxiliary gas.
The plasma gas arc melts the metal, while the auxiliary gas cools all parts of the cutting torch and blows away the melted metal.
2. The cutting power supply includes two parts: main circuit and control circuit.
Electrical principle: the main circuit includes contactor, three-phase power transformer with high leakage reactance, three-phase bridge rectifier, high-frequency arc striking coil and protective elements.
High leakage reactance leads to steep external characteristics of power supply.
The whole process of cutting arc starting is controlled by the high-frequency switch of cutting arc starting.
3. The power supply of the main circuit is controlled by the contactor;
The opening and closing of gas is controlled by solenoid valve;
The control circuit controls the high frequency oscillator to ignite the arc, and stops the high frequency after the arc is established.
Plasma arc cutting process parameters
Various plasma arc cutting process parameters directly affect the stability, cutting quality and effect of the cutting process.
The main cutting specifications are briefly described as follows:
1. No load voltage and arc column voltage
Plasma cutting power supply must have enough high no-load voltage to easily start the arc and make the plasma arc burn stably.
The no-load voltage is generally 120-600v, while the arc column voltage is generally half of the no-load voltage.
Increasing the arc column voltage can significantly increase the power of plasma arc, so it can improve the cutting speed and cut metal plates with greater thickness.
The arc column voltage is often not achieved by adjusting the gas flow and increasing the internal shrinkage of the electrode, but the arc column voltage cannot exceed 65% of the no-load voltage, otherwise the plasma arc will be unstable.
2. Cutting current
Increasing the cutting current can also improve the power of plasma arc, but it is limited by the maximum allowable current, otherwise it will thicken the plasma arc column, increase the slit width and reduce the electrode life.
3. Gas flow
Increasing the gas flow can not only improve the arc column voltage, but also enhance the compression effect on the arc column, so that the plasma arc energy is more concentrated and the jet force is stronger, so the cutting speed and quality can be improved.
However, if the gas flow is too large, the arc column will be shortened, the heat loss will increase, and the cutting ability will be weakened until the cutting process cannot be carried out normally.
4. Electrode shrinkage
The so-called internal shrinkage refers to the distance from the electrode to the end face of the cutting nozzle.
The appropriate distance can make the arc get good compression in the cutting nozzle and obtain the plasma arc with concentrated energy and high temperature for effective cutting.
If the distance is too large or too small, the electrode will be seriously burned, the cutting nozzle will be burned out and the cutting capacity will be reduced.
The internal shrinkage is generally 8-11mm.
5. Cutting nozzle height
The cutting nozzle height refers to the distance from the end face of the cutting nozzle to the surface of the workpiece to be cut.
The distance is generally 4 ~ 10mm.
It is the same as the internal shrinkage of the electrode.
The distance should be appropriate to give full play to the cutting efficiency of plasma arc, otherwise the cutting efficiency and cutting quality will be reduced or the cutting nozzle will be burned out.
6. Cutting speed
The above factors directly affect the compression effect of plasma arc, that is, the temperature and energy density of plasma arc.
The high temperature and high energy of plasma arc determine the cutting speed, so the above factors are related to the cutting speed.
On the premise of ensuring the cutting quality, the cutting speed should be improved as much as possible.
This not only improves the productivity, but also reduces the deformation of the cut parts and the heat affected area of the slit area.
If the cutting speed is not appropriate, the effect is opposite, and the slag will increase and the cutting quality will decrease.