Plasma cutting torch (also known as Plasma arc cutting gun) is a key component for generating and cutting plasma arc. Figure 2-27 shows its general structure. It is mainly composed of cutting torch body, electrode assembly, nozzle and pressure cap. The manual cutting torch has a handle. The specific structure varies with the type of working gas, gas flow mode and electrode used. Under the premise of ensuring reliable work, the cutting torch structure should be as small as possible to reduce weight and make the operation convenient. The designed cutting torch shall meet the following requirements:
(1) The electrode and nozzle should be strictly centered, that is, the concentricity should be high, in order to improve the cutting effect.
(2) Electrode and nozzle can get good cooling, prevent overheating damage.
(3) The upper and lower cavities should be reliably insulated, and the connection should be stable and reliable.
(4) Water and ventilation parts and connections shall be watertight and airtight, and leakage shall not occur.
(5) The position and size of the air inlet should be matched with the selected electrode and nozzle to obtain a stable and well-compressed plasma arc.
1 cut torch cap; 2 electrode collet; 3 one electrode; 4. 12-O ring; 5. Working gas inlet pipe;
6. Cooling water drain pipe; 7. Cut the cable; 8 small arc cable; 9 Cooling water inlet pipe;
10 – Cutting torch; 11 a pair of middle blocks; 13 water-cooled nozzle; 14 pressure cap
There are two basic forms of gas flow in plasma cutting torch: axial flow and vortex ,See Figure plasma cutting torch-1
Working gas flow mode in plasma cutting torch
(a) axial flow (b) vortex
The nozzle is the core part of the plasma arc cutting torch. Its structure and geometric size play an important role in the compression and stability of the plasma arc, and directly affect the cutting ability, notch quality and nozzle life. The geometry of the nozzle is shown in Figure Plasma cutting torch-2.
Nozzle and cutting torch internal cavity geometry
D–Gas chamber diameter; H–Total height of one air chamber; L — height of air inlet; d– Nozzle aperture;
d0— Intake aperture; l– Length of compression channel; α–compression Angle
Nozzle passage is the main “pass” for arc compression. The smaller the nozzle aperture and the larger the channel length, the stronger the compression effect on the plasma arc, the more concentrated the energy, the stronger the cutting ability and the higher the cutting quality. But when the nozzle aperture is too small and the hole length is too long, the plasma arc is unstable, and even can not lead to arc, and easy to produce double arc (that is, there is not only an arc between the electrode and the workpiece, but also the arc through the nozzle), so that the nozzle burns. In order to prevent serious burning of nozzles, the allowable limit current of nozzles with different aperture is determined .
Practice has proved that it is suitable when the ratio of nozzle aperture to compression channel length is 1:(1.5-1.8). Under common specifications, the nozzle aperture is 2.4~4mm, and the length of the compression channel is 4.0~7.0mm. The nozzle aperture depends on the type of gas, current and thickness of cutting material.
The compression Angle α mainly affects the degree of arc compression. When the compression Angle is small, the arc compression is strengthened, but too small will make the arc unstable, and it is easy to cause arc between the electrode and the nozzle because the distance between the cavity wall and the electrode is too close. It is proved that the plasma arc is stable and has good compression and cutting ability when α is about 30°.
Air inlet height L and air inlet aperture d0 have influence on cutting ability and cutting quality. The test results show that the air inlet height L=12~14mm is suitable when the air chamber diameter D=12mm and the air chamber height H=33~37mm. Tangential air inlet should dip slightly, no up dip is allowed. The inlet aperture d0 should be 4-5mm, otherwise it will cause turbulence and affect the stability of plasma arc.
The nozzle is generally made of copper with good thermal conductivity. Figure Plasma cutting torch-3 shows the structure of the nozzle. The wall thickness of the nozzle is 1 to 2mm. Too thick wall, poor water cooling effect; Too thin, low heat capacity, easy to burn. The surface roughness of nozzle orifice should be below Ra1.6µm. In order to reduce the double arc factor, there should be a smooth transition of small arc R2 or 0.5mm x 45° chamfer at the exit of the orifice.
Nozzle structure shape
R1–a rounded corner; R2– a chamfering; d– a nozzle aperture
α – compression Angle; l — Length of compression channel
The shape of the nozzle, especially the shape of the end, manufacturers have their own technical know-how. Generally there are several shapes as shown inFigure Plasma cutting torch-4. The outlet section in Figure Plasma cutting torch-4A is in straight barrel shape, which is characterized by strong thermal shrinkage effect and easy to keep the long and thin state of plasma flow after ejection. The lower part of the outlet section in Figure Plasma cutting torch-4B is shaped like a flap to avoid double arcs. The shape of the outlet section shown in Figure Plasma cutting torch-4C is taken to mitigate the eccentricity of the plasma flow caused by the poor alignment of the electrode and nozzle hole. Once the inclination Angle of the upper section is too large, the plasma flow is easy to disperse due to expansion after ejection. Figure Plasma cutting torch-4A and B are usually used.
The basic structure types of plasma arc gas cutting nozzles are shown in Figure plasma cutting torch-5
The specific structure of plasma arc cutting torches varies from manufacturer to manufacturer. The common manual plasma arc cutting torches are shown in Figure Plasma cutting torch-5 to Figure Plasma cutting torch-9
Figure Plasma cutting torch-6 shows the schematic diagram of cutting torch for LG-400-1 plasma arc cutting machine. The concentricity of the electrode and nozzle of the cutting torch is guaranteed by the machining accuracy of the cutting torch parts. There is no special adjustment mechanism, which is usually called the central non-adjustable cutting torch.
Figure Plasma cutting torch–7 shows the structure diagram of the center adjustable cutting torch for LG-500 cutting machine. Because the concentricity of electrode and nozzle can be adjusted flexibly, the machining accuracy of parts and the flatness of electrode are required to be low.
Figure Plasma cutting torch-8 shows the structure of the cutting torch for epoxy resin casting. The insulation layer of the upper and lower cavities and the cutting torch shell are made of epoxy resin material, which not only has good insulation performance, but also greatly simplifies the processing process.
Figure Plasma cutting torch-9 shows the structure diagram of the high-current air plasma arc cutting torch. The applicable working current is 250~300A. The electrode (size: 2.5mm×7mm) is embedded in the structure of pure copper base, and the working airflow is vortex type. The nozzle aperture is 3mm, the channel length is 4mm, and the distance from the electrode end to the end of the nozzle is 10mm.
When the current is less than 100A, the torch is mostly air-cooled, so the structure can be greatly simplified.