Electrical Wire Manufacturing

Electrical wire and pipe manufacturing has developed from as early as 1797, when Joseph Bramah was granted the first patent for making pipes out of lead and other soft metals. As man's needs for electrical wire grew, so did the technology. Over the next hundred years, new manufacturing techniques enabled hydraulic power presses to force pre-heated chunks of metals (copper, aluminum, brass, lead) called billets through various forms to produce extruded products. These early produced products include pipes, rods, wires and various other shapes. In today's time, other materials are extruded through forms such as magnesium, molybdenum, nickel, titanium, uranium, zirconium, and super alloys.

There are many proven modern manufacturing techniques available to form metal products. Some of the most common methods used by today's manufactures are radial extrusion, impact extrusion, cold draw, forward extrusion, hydrostatic continuous extrusion, warm extrusion and hot extrusion. Of these, hot extrusion is the preferred choice to produce electrical wire.

Hot extrusion has many advantages over the other techniques for wire production. The primary alloys used for electrical conductors are copper or aluminum based. With these soft alloys, this process provides an infinite number of possibilities for parallel surface applications. Grain microstructures improve due to the high heat and pressure needed for hot extrusion. Material flexibility and strength properties are well suited for electrical conductors after this process, but these properties can easily be changed by heat treating or cold working. This method of production is an economical way to produced wires because it is easily produced through high volume production and capable of maintaining tight tolerances. Tolerance fluctuations are a function of the size and geometry of the work piece, the type of tooling used and difficulty of the extruded part. Another advantage is the relatively low start up costs. Tooling cost are nominal and the set up cost are lower than rolling a similar product. Design changes are easily made with extruding methods with little increases in costs and is ideal for small orders.

A disadvantage to hot extrusion are defects. They happen in all forms of extrusion. Typically, defects show up as non-uniform flow of material from the billet through the die. These defects are caused by funnel-shaped hollow spots that formed in the center of the billet, and surface defects such as scaling, blistering, and die lines. To minimize oxide deposits from forming on the billets, they are usually heated in an inert atmosphere. Oxidation will change the friction properties of the material and will require a higher force and decreases die life. Oxidation problems will change the dimensions of the product and increase surface defects.

There are two techniques widely used for hot extrusion, machinery is available for both techniques to extrude in a vertical or a horizontal position. Direct extrusion uses a reusable dummy block to make direct contact the billet on one side and the ram on the other. Pressure apply by the ram is usually generated from hydraulic or steam energy. For copper alloys, the die and billets are preheated to 600-1000 degrees Celsius and 300-600 degrees Celsius for aluminum alloys. This method is the most common for hot extrusion. Up to six hundred ninety mega Pascal are needed to extrude most copper alloys used today. Part of the energy must be use to over come the frictional force between the billet and the container. These forces are increased as the billets are upset to fill the containers. The ram pressure will peek just before the material extrudes through the die. This critical pressure is known as the breakthrough force. After the critical pressure has been reached and the material flows from the die, the pressure needed to continue the flow will decreased. As the billet gets smaller the pressure needed to maintain the rate of flow also diminishes until a minimum is reached. At the minimum pressure, an increase of pressure is need to maintain the flow rate from the die. This pressure increase is due to the small amount of material left in the chamber that must rapidly flow to the die opening to maintain the extruded flow rate.

Indirect extrusion, or also commonly known as backward extrusion, the billet does not move relative to the chamber walls. Instead the die is pushed to the billet using a hollow stem ram or a channeled shaft ram. The extruded material extrudes through the die and is channeled through the shaft. Because the billet is stationary relative to the chamber walls, there is no frictional forces to overcome between the two surfaces. The result is a 25-30% reduction in critical pressure needed for indirect extrusion compared to direct. The forces required for extrusion after the breakthrough pressure is achieved remains constant throughout the extrusion cycle of the billet. This yields a uniform pattern flow and allows for faster extrusion rates. This is a cost saving advantage over direct extrusion. Lower pressures increases die life, tooling and decrease energy consumption. Other advantages include larger billet sizes, less surface cracking due to lower frictional forces. Disadvantages are surface defects on the billet propagate to the extruded product. Because of this problem, this method can not be used for many architectural purposes or anodizing. Many indirect extruded products have to be wire brushed, which increases production costs. This is avoided for direct extrusion because these surface defects do not get extruded through the die; they are removed from the billet chamber after every cycle. Indirect extrusion is not suited for all products.

After the extruding is accomplished, the wire is cooled and insulated. Thermal aging is a measure of the useful life of insulation; how long will hold it's insulating properties. The insulation life can be greatly shorten if thermally damaged. Thermal properties and thermal conductivity are important factors in choosing a insulation covering. Each type of insulating material has a different manufacturing application. Insulating liquids such as oils and chlorinated hydrocarbons are used in cables circuit breakers, capacitors and transformers. Typical insulation types are enamels, papers, film materials, coating powders, varnishes, mica composites, laminates, plastics, ceramics and organic solids.

Bibliography

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"Wiredrawing Machinery Plant...", Beijing Review, Oct. 17,1994, p30-31

Kalpakjian. "Extrusion and Drawing." Manufacturing Engineering and Technology. p417-441.

Wick, Benedict, Veileux. "Drawing, Extruding and Upsetting." Tool and Manufacturing Engineer Handbook Vol. 2, Forming. C1984