Structure of the Plasmatron Basic Units

Taking into account that the processing of metals are preferably used with a DC plasma torches rod nonconsumable (non-consumable) electrode, this section will be discussed in detail the design features of this type of plasma torches.

The main components are considered plasmatron cathode, nozzle assembly and twist the working gas. The latter provides the best conditions for the formation of the arc column from all other circuits stabilize the arc, presented in the previous section, so it is discussed in more detail. The core non-consumable electrodes, cathodes made in the form of copper-cooled bearing housing with the cathodic insertion of tungsten, zirconium and other refractory metals and their alloys.

The most common embodiment of the cathode - the fixing of a tungsten rod of the split collet. Tsang made of resilient material elektroprrvodnogo (brass, bronze Braze), since it is a current-carrying part and should provide a reliable contact with the cathode and the heat from it. There are two types of collets: some press out electrode is pressed into its hole, slightly smaller than its diameter, the other - an electrode is inserted freely, and the collet is crimped on the outside by drawing it into the head of the torch body with a threaded connection. The second type of collet is more rational, as this provides better alignment of the electrode and tighter compression due to its efforts in the threaded connection. It is better to solve the problem sealing head torch. When Zahn govom clamp electrode provides the ability to move as it is eroded, replace it with light. Therefore, this relatively simple embodiment of the electrode assembly is widely used in hand-held plasma cutters, as well as plasma torches for manual and mechanized welding, working in small and medium currents. By increasing the load current to the cathode is not provided sufficient heat through the tungsten rod to the point of contact with its collet. When the arc current above 300 and the cathode collet design is complicated, and their reliability during operation is reduced. In addition, the chuck completely acceptable for zirconium electrodes, which require more intense heat directly to the location of the cathode spot. Most current load at a much lower erosion, provide cathodes, which are mechanically insert firmly and fixedly connected to a water-cooled copper holder. In this case soldered tungsten paste on the silver solder or welded by diffusion bonding in vacuum, and zirconium, as a plastic, pressed into a copper holder. In all subsequent versions of the copper bearing housing is made as a hollow cylinder with inserted inside the tube through which cooling water is supplied, washing the inner cavity of the body. According to Scheme 2 (Fig. la) cathode paste is strengthened in the whole body, schemes 3 and 4 - to the ends of the copper plug, connected to the housing, respectively, planting or conical thread. The best scheme is 5 (Fig. a). Replacement tip in this case is made with the least expenditure of material, easily stamped, attached to the body with the help of nut. All threaded connections with the body of the electrode tip should be compacted with rubber rings. These connections are more reliable than the connection to the dense conical landing. When you insert through the cathode (Scheme 5, Fig. A) provides the best heat dissipation from the cathode. This scheme is used in the manufacture of electrodes with zirconium cathode, and scheme 2 (Fig. la), with some flying inserts used in the manufacture of electrodes with a tungsten cathode. Ways of further improving the design of rod cathodes and search for new cathode materials are far from exhausted. In this direction, continuing a thorough investigation. Forming the nozzle is the most heat-stressed elements of torches, and therefore require careful structural embodiment. As shown by long experience of operation of plasma torches, the best material for the manufacture of nozzles is copper of high purity (grade Ml, MO), which has high thermal conductivity. Reinforcement of the nozzle wall heat resistant, heat-conducting materials are generally less leads to a decrease in its lifetime. Nozzles maloamper GOVERNMENTAL and manual plasma torches can be made with natural or gas (air) cooling (Scheme 1, 2, Fig. B). In most cases, use water-cooling nozzles, and the nozzle portion in direct contact with the plasma column, runs interchangeable. The diagrams 3, 4 and 5 (Fig. lb) are the most common types of fastening the nozzle with a lid (screw mount, seal with rubber pads, nipple). The best replacement is a nipple connection nozzle, made in the form of "glasses", fixed and compacted without rubber gaskets with stainless steel lid. Nozzle and the lid are made by stamping. With this design provided the best conditions for heat removal from the nozzle. Knot twist of the working gas determines the quality of stabilization of the plasma arc column. Structural variants of twist knots can be divided into four main groups, each of which can perform functions swirler torch body, a ceramic plate, placed at the entrance to the nozzle, the nozzle itself and the electrode of the plasma torch. In Schemes 1 and 2 (Fig. c) the working gas is fed into the chamber through one or a few tangential holes drilled in the hull of the plasma torch. Several holes provide greater uniformity of distribution of gas in a circle, but the complicated system of gas supply to the torch.

In the washer-swirler provides a good quality of the formation of the post, but it is susceptible to destruction by heat of the arc column and requires great precision during assembly of the plasma torch. In the swirl nozzle-gas is fed through the tangential-axial channel, profrezerovannye on its inner surface. Especially interesting option dvuhsoplovoy forming system (Scheme 4, Fig. B) with a double swirling gas flow. It is possible to dramatically reduce the length of the nozzle channel, replacing most of its rings stable gas rotating around the pole arc. The disadvantage of this type of nozzle, the swirler is the complexity of its structure and, consequently, the difficulty of its manufacture. Nozzle, as the most frequently wears removable part of the plasma torch should be simple to manufacture. The most successful feature rotator performs an electrode on the outer surface of the screw thread which is executed (Scheme 5, Fig. C). In a dense planting of such an electrode in the case of the plasma torch working gas enters the chamber of grooves formed by the screw thread. At the same time without substantially complicating the hull design of the electrode provides a high quality gas-vortex stabilization. To make a constructive analysis of the major components of the plasma torch was complete, we consider some features of the junction of these nodes together. A rigid mechanical connection of the electrode and nozzle unit by means of an insulator. In choosing the material and design of the insulator should be aware that it must withstand the high voltage oscillator, while maintaining the mechanical strength and density at high temperature and humidity, in addition, he has a good yield accurate machining. The most fully satisfies the requirements of this epoxy resin, which has high insulation properties (breakdown voltage for it is 20-30 kV / mm) and good adhesion to the metal retains its properties at elevated temperatures (up to 500 ° C), resistant to moisture and various aggressive vapors and gases. The insulator can be made ​​of heat-resistant fiberglass-type AG-4S, somewhat inferior to the properties of epoxy resin. The main disadvantage of this material is the lack of adhesion to the metal parts of the plasma torch. Insulators are made ​​of these materials using molding or extrusion. This requires special tooling, so the use of these materials is justified only when the serial production of plasma torches. In the manufacture of single torches appropriately carved out of hard rubber bushing or Teflon. However, these materials are inferior to the above for its insulating and strength properties. One of the conditions for a stable arc is the exact alignment of the electrode and the nozzle assembly at the plasma torch. The deviation of a few tenths of a millimeter can lead to the formation of a double arc. High precision alignment of the electrode and the nozzle can be achieved only with rigid fixation of their relative positions. It is therefore not recommended to design the plasma torches with movable electrodes, for example, to excitation of the arc by shorting the electrode to the nozzle. Usually in the plasma torches for mechanized handling of the arc is used to excite the oscillator, which provides the excitation gap in the arc between the electrode and nozzle is not more than 4-5 mm. In the hand-held plasma torches can be used a graphite rod, which is briefly introduced in the nozzle, and, closing the gap electrode - nozzle, exciting duty arc. However, this method of excitation leads to increased wear of the electrode and nozzle. An important condition for a stable arc plasma torches in direct action is the fulfillment of certain size ratio between the diameter of the cathode dK, nozzle diameter dc and nozzle channel length lc. Experimentally, use of plasma torches is established that the possibility of double arcing is eliminated under two conditions: dc ^ dK and lc ^ dc. The value of the diameter of the cathode, as already noted, is determined by the arc current. In the indirect action of the plasma torches, in which the phenomenon of double toryh possible arcing, the channel length can not exceed the nozzle with a diameter and a half to two times or more. Excessive oversizing of the phenomenon is limited to shunting arc. These dimensional relations are the starting point for the design of plasma torches. If you select other sizes of the plasma torch should first start with his appointment. For example, the weight and dimensions of the plasma torch, designed for mechanical process, not as limited as compared with the weight and size of hand-held plasma torches.

See also:
Plasma Welding
Plasma Welding Introduction
Plasma Welding Technique
Microplasma Welding
Gases for plasma processing of materials
Separation of plasma jet cutting
Compression of the arc
The energy properties of the plasma arc
Rationalization of plasma welding
Plasma welding and spraying
The plasma melting and remelting
Plasmatron. Requirements for plasmatron
Plasmatron. Schemes, classification
Classification by type of electrode plasma torches
Classification of torches by the nature of the current
Structure of the plasmatron basic units