Plasma Cut – What Is Plasma Cutting

Plasma cutting with different working gases can cut all kinds of metals that are difficult to cut by oxygen, especially for non-ferrous metals (stainless steel, aluminum, copper, titanium, nickel). The main advantage is that the cutting thickness of metals is not large. At this time, plasma cutting speed is fast, especially when cutting ordinary carbon steel sheet, the speed can reach 5-6 times that of oxygen cutting method, the cutting surface is smooth, the thermal deformation is small, and the heat-affected zone is less.

Plasma cutting machines are widely used in automobiles, locomotives, pressure vessels, chemical machinery, nuclear industry, general machinery, engineering machinery, steel structures, ships and other industries.

Plasma arc cutting is a processing method that uses the heat of a high-temperature plasma arc to locally melt (and evaporate) the metal at the incision of the workpiece, and use the momentum of the high-speed plasma to remove the molten metal to form an incision.

The Historical Evolution Of Plasma Cut

Traditional plasma cutting in the 1950s

Since the plasma arc process in the mid-1950s, considerable research has been devoted to increasing arc compression without producing double arcs. The plasma arc cutting used in that period is now called “traditional plasma cutting”. If the user is cutting various types of metals with different thicknesses, traditional plasma cutting will be very cumbersome. For example, using traditional plasma processes to cut stainless steel and aluminum requires different gases and gas flow rates to obtain the best cutting quality on both metals. Although traditional plasma cutting dominated from the 1950s to the 1990s, this process often requires very expensive argon-hydrogen gas mixtures.

Dual Airflow Technology in the 1960s

The dual gas flow technology was invented in the 1960s, and it added another shielding gas around the plasma nozzle. Generally, in dual-air operation, the cutting gas or plasma gas is nitrogen or argon-hydrogen mixture, and the shielding gas is selected according to the metal to be cut. The typical shielding gas used when cutting mild steel uses air to cut stainless steel CO is used when cutting aluminum, and argon-hydrogen gas is used when cutting aluminum. This technology cuts carbon steel faster than flame cutting. The main advantage of this method is that the nozzle can be hidden in the ceramic gas hood or protective cover, preventing the nozzle from contacting the workpiece, reducing the tendency of “double arc”, the protective gas covers the cutting area, and improving the cutting quality and cutting speed. It can also cool the nozzle and the shield. Air plasma cutting was introduced in the early 1960s for cutting carbon steel. The exothermic reaction of oxygen in the air with the molten steel plate can provide additional energy.

Oxygen plasma cutting in the 1970s and 1980s

In the early 1970s, an industrially available hafnium and zirconium were discovered, which can resist the rapid burning of electrode materials caused by high temperature in oxygen plasma arc cutting. Oxygen has aroused great interest as a plasma gas. After that, oxygen as a plasma gas became possible, and the application of oxygen plasma to carbon steel cutting became the latest development of plasma arc cutting technology. Oxygen plasma cutting meets the conditions required for cutting with a large range, no dross, and high cutting speed. When operating at a lower current level, the cutting speed is greatly increased, and the cutting edge is smooth, square and softer. This cut edge is easier to bend or weld. All steel plates, including high-hardness and low-alloy steels, can be cut basically without dross using this new technology.

Modern high-precision plasma cutting and its intelligent development

In the early 1990s, the “fine plasma” concept entered the market, challenging the laser market for the first time. Laser cutting is an important competitor of plasma cutting in the metal cutting industry because of its ability to produce high-quality cuts while protecting accurate accuracy. Plasma equipment manufacturers have increased their design efforts in order to further improve the cutting quality of their equipment. By greatly reducing the size of the nozzle hole and generating an extremely compressed arc, plasma cutting achieves the high energy density required to compete with laser products. The fine plasma system has become an advanced plasma product that competes with lasers in the metal cutting industry.

The Cutting Equipment Of Plasma Cutting

The plasma cutting system is mainly composed of air supply device, water device, power supply and cutting gun. The water-cooling gun also needs to have a cooling circulating water device.

• (1) Air supply device The main equipment of the air supply device for air plasma arc cutting is an air compressor larger than 1.5kw, and the gas pressure required for cutting is 0.3-0. 6MPa. If other gases are used, the bottled gas can be used for cutting after decompression.
• (2) Power supply Plasma cutting adopts a DC power supply with external characteristics of steep drop or constant current. In order to obtain satisfactory arc ignition and arc stabilization effects, the no-load voltage of the power supply is generally twice the arc voltage. The no-load voltage of the commonly used cutting power supply is 350-400V.
• (3) Cutting torch The specific form of the cutting torch depends on the current level of the cutting torch. Generally, torches below 60A use air-cooled structure; and torches above 60A use water-cooled structure. The electrode in the cutting gun can be pure tungsten, thorium tungsten, bell tungsten rod, or mosaic electrode. The electrode material is preferably cast tungsten.

Plasma cutting has the advantages of large cutting thickness, flexible cutting, simple clamping of the workpiece and the ability to cut curves. It can be widely used in cutting all metal materials and non-gold scrap materials.

The Method Of Plasma Cutting

In addition to the general forms of plasma cutting methods, there are also derived forms such as water compression plasma cutting. The most commonly used methods are general plasma cutting and air plasma cutting.

General cutting

In general plasma cutting, shielding gas is not used, working gas and cutting gas are ejected from the same nozzle. When the arc is started, a small flow of ionic gas is sprayed as the ionizing medium; when cutting, a large flow of gas is sprayed at the same time to remove the molten metal.

Air cutting

Air plasma cutting generally uses compressed air as ion gas. This method has low cutting cost and convenient air source. Compressed air is heated, decomposed and ionized in the arc, and the generated oxygen cuts the metal to produce a chemical exothermic reaction, which accelerates the cutting speed. The fully ionized air plasma has a high enthalpy value, so the arc energy is large and the cutting speed is fast.

Working Gas

The development of plasma cutting to the present, the available working gas (working gas is the conductive medium of the plasma arc, it is also the heat-carrying body, and at the same time the molten metal in the incision must be excluded), the cutting characteristics of the plasma arc and the cutting quality and speed are all Obvious impact. Commonly used plasma arc working gases are argon, hydrogen, nitrogen, oxygen, air, water vapor and some mixed gases.

Cutting specification

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:

No-load voltage and arc column voltage

The plasma cutting power supply must have a high enough no-load voltage to easily start the arc and make the plasma arc burn stably. The no-load voltage is generally 120-600V, and the arc column voltage is generally half of the no-load voltage. Increasing the arc column voltage can significantly increase the power of the plasma arc, thereby increasing the cutting speed and cutting larger thickness metal plates. The arc column voltage is often 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.

Cutting current

Increasing the cutting current can also increase the power of the plasma arc, but it is limited by the maximum allowable current, otherwise the plasma arc column will become thicker, the slit width will increase, and the electrode life will decrease.

Gas flow

Increasing the gas flow can not only increase the arc column voltage, but also enhance the compression of the arc column, so that the plasma arc energy is more concentrated and the jet force is stronger, thus improving the cutting speed and quality. However, if the gas flow is too large, it will shorten the arc column, increase the heat loss, and weaken the cutting ability until the cutting process cannot be performed normally.

Electrode shrinkage

The so-called shrinkage refers to the distance between the electrode and the end surface of the cutting nozzle. A proper distance can make the arc be well compressed in the cutting nozzle, and obtain a 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 tip will be burnt out and the cutting ability will be reduced. The shrinkage is generally 8-11mm.

Cutting nozzle height

The height of the cutting nozzle refers to the distance from the end 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, and the distance must be appropriate to give full play to the cutting efficiency of the plasma arc, otherwise the cutting efficiency and cutting quality will be reduced or the cutting nozzle will burn out.

Cutting speed

The above various factors directly affect the compression effect of the plasma arc, that is, the temperature and energy density of the plasma arc. The high temperature and high energy of the plasma arc determine the cutting speed, so the above various factors are related to the cutting speed. On the premise of ensuring the cutting quality, the cutting speed should be increased as much as possible. This not only improves productivity, but also reduces the amount of deformation of the cut parts and the heat-affected area in the kerf area. If the cutting speed is not appropriate, the effect is opposite, and the sticking slag will increase and the cutting quality will decrease.

Security

• 1. The lower part of the plasma cutting should be equipped with a water tank, and the cutting part should be cut under water during the cutting process to avoid the smoke that is harmful to the human body.
• 2. During the plasma arc cutting process, avoid direct visual observation of the plasma arc, and wear professional protective glasses and face shields to avoid burns to the eyes and skin by the arc.
• 3. During the plasma arc cutting process, a large amount of toxic gas will be generated, which requires ventilation and wear a multi-layer filter dust mask.
• 4. In the plasma arc cutting process, wear towels, gloves, foot guards and other protective equipment to prevent the skin from being burned by the splashing sparks.
• 5. In the plasma arc cutting process, the high frequency and electromagnetic radiation generated by the high frequency oscillator can cause damage to the body, and some long-term practitioners even have symptoms of infertility. Although the medical field and the industry are inconclusive, they still need to protect themselves. jobs.