Extrusion(redirected from Screw extruder)
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a method of pressure working of metals in which the metal is squeezed from a closed chamber through a die whose shape and dimensions determine those of the extruded section. High hydrostatic pressure is generated during extrusion, resulting in a significant increase in the plasticity of the metal. Extrusion may be used in working many brittle materials, which cannot be worked by methods such as rolling, forging, or drawing. A distinction is made between direction extrusion, in which the direction of motion of the metal is the same as the direction of motion of the dummy block, and backward, or indirect, extrusion, in which the metal flows in the direction opposite to the motion of the die, which also acts as the dummy block.
During direct extrusion of a solid section, the ram transmits pressure to the billet in the chamber through the dummy block. The metal of the billet is thereupon extruded through the opening in the die, which is mounted in a die holder, and a section is produced. The rate of outflow of the section exceeds the rate of advance of the ram by the same factor as the cross-sectional area of the chamber exceeds the area of the die opening. The ratio of these areas is called the elongation coefficient. In direct extrusion of pipe, the metal of the billet is extruded through an annular slit between the die and a mandrel, forming a pipe of the desired configuration. In this case the billet moves relative to both the chamber and the mandrel.
In the case of backward extrusion, the force is applied to the billet through the chamber, which moves in the direction indicated by the arrow through a shortened ram or plug, which closes off the chamber. At the other end, the chamber is closed off by an elongated die holder, in which the die is mounted. When the chamber moves, the billet moves with it, and the metal is extruded into the channel of the die, thus forming a section.
Friction of the metal against the walls of the chamber during direct extrusion leads to significantly higher shear stresses in the outer layers of the billet than in the inner layers. The nonuniformity of stress leads to differences in structure and properties over the cross section of the article. This is especially noticeable in the extrusion of large-diameter rods. There is no such friction during backward extrusion, which results in a much smaller nonuniformity of structure and properties over the cross section. In addition, the reverse outflow requires considerably less force, which makes it possible to reduce the temperature of the blank and to increase the speed of the process.
Extrusion with welding is also used in the production of pipes and hollow sections from aluminum and magnesium alloys, and in some cases from copper and titanium alloys. Under pressure transmitted by the ram, the billet is split by the peak of the die into two or more streams of metal, depending on the design of the die. The streams are then fused under high pressure surrounding the mandrel in a solid mass; the mandrel and the peak of the die are fabricated from a single piece. The final shaping of the pipe takes place in the annular slit between the die and the mandrel.
Other methods of extrusion include production of pipe from solid billets with preliminary piercing by a mandrel, production of solid and hollow sections with continuously changing or stepped variable cross section, and production of wide ribbed sheets, or panels, from slit chambers.
Hydroextrusion, in which the pressure on the billet is transmitted through a fluid, is also used in industry. The field of force in this method is generated by a fluid under high pressure that is fed into the chamber from an external source or by pressure exerted on the liquid by a sealed ram. The pressure achieved in modern installations for hydroextrusion is about 3 giganewtons per sq m, or 30,000 kilograms-force per sq cm.
Extrusion may be performed with or without preheating of the billets and tools. Cold extrusion (that is, extrusion without preheating) is used in processing readily deformable metals, such as lead, tin, and pure aluminum. Through the very high pressures and the absence of friction against the chamber walls, cold hydroextrusion also makes possible the processing of metals and alloys that are more difficult to deform (Duralumins, copper alloys, and steels). Hot extrusion is used to produce articles from various metals and alloys—for example, aluminum, titanium, copper, and nickel alloys, and also refractory metals. Billets are preheated to the highest temperatures (1600°-1800°C) in the extrusion of tungsten and molybdenum.
Extrusion is performed in horizontal hydraulic presses and, less frequently, in vertical hydraulic presses, which are mainly used for pipe extrusion and hydroextension. In some cases, cold extrusion of pipes from readily deformable metals is performed in power presses. Extrusion makes possible the production of solid sections with cross-sectional areas of 0.3-1500 sq cm and diameters of circumscribed circles of 1.5-90 cm, round rods 0.6-60 cm in diameter, and pipes 0.8–120 cm in diameter, with a wall thickness of 0.1–10 cm.
REFERENCESPerlin, I. L. Teoriia pressovaniia metallov. Moscow, 1964.
Prozorov, L. V. Pressovanie stali i tugoplavkikh splavov, 2nd ed. Moscow, 1969.
Zholobov, V. V., and G. I. Zverev. Pressovanie metallov, 2nd ed. Moscow, 1971.
Ermanok, M. Z. Proizvodstvo polykh profilei iz aliuminievykh splavov pressovaniem so svarkoi Moscow, 1972.
Kolpashnikov, A. I., and V. A. Vialov, Gidropressovanie metallov. Moscow, 1973.
M. Z. ERMANOK
the volcanic ejection of molten lava, which forms a volcanic dome above the volcanic vent. Gases and ash are periodically emitted from the dome with great force.
The forcing of solid metal through a suitably shaped orifice under compressive forces. Extrusion is somewhat analogous to squeezing toothpaste through a tube, although some cold extrusion processes more nearly resemble forging, which also deforms metals by application of compressive forces. Most metals can be extruded, although the process may not be economically feasible for high-strength alloys.
The most widely used method for producing extruded shapes is the direct, hot extrusion process. In this process, a heated billet of metal is placed in a cylindrical chamber and then compressed by a hydraulically operated ram (see illustration). The opposite end of the cylinder contains a die having an orifice of the desired shape; as this die opening is the path of least resistance for the billet under pressure, the metal, in effect, squirts out of the opening as a continuous bar having the same cross-sectional shape as the die opening. By using two sets of dies, stepped extrusions can be made.
The extrusion of cold metal is variously termed cold pressing, cold forging, cold extrusion forging, extrusion pressing, and impact extrusion. The term cold extrusion has become popular in the steel fabrication industry, while impact extrusion is more widely used in the nonferrous field.
The original process (identified as impact extrusion) consists of a punch (generally moving at high velocity) striking a blank (or slug) of the metal to be extruded, which has been placed in the cavity of a die. Clearance is left between the punch and die walls; as the punch comes in contact with the blank, the metal has nowhere to go except through the annular opening between punch and die. The punch moves a distance that is controlled by a press setting. This distance determines the base thickness of the finished part. The process is particularly adaptable to the production of thin-walled, tubular-shaped parts having thick bottoms, such as toothpaste tubes.
Advantages of cold extrusion are higher strength because of severe strain-hardening, good finish and dimensional accuracy, and economy due to fewer operations and minimum of machining required. See Metal forming