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a machine for softening materials and shaping them by squeezing through a die (an extrusion head, or nozzle) that has a cross section corresponding to the desired shape of the product. The processing of materials with an extruder is known as extrusion.
Extruded products are chiefly made from thermoplastics; rubber stock may also be used (in this case the extruder is often called a shprits-mashina in Russian). Extruders are used to manufacture films, sheets, tubing, hoses, products with complex shapes, and other items as well as to apply a thin coating to paper, cardboard, cloth, and foil and to insulate wires and cables. They may also be used to produce granules of materials, prepare compositions for calendering, and form metal products.
An extruder consists of several basic subassemblies: a housing equipped with heating elements, a working member (a screw, disk, or piston) located inside the housing, a feed assembly for the material being processed, a power drive, a system for setting and maintaining temperatures, and other monitoring, measuring, and regulating devices. Extruders are classified according to the type of working member or members as single-screw and multi-screw, disk, piston (plunger), and other types.
The first extruders were designed in the 19th century in Great Britain, Germany, and the USA for the application of guttapercha insulation to electric wires. They were put into series production in the early 1900’s and were first used to process plastics circa 1930. Between 1935 and 1937 electric heating of the housing replaced steam heating, and between 1937 and 1939 extruders appeared equipped with a longer screw (the prototype of the modern type) and the first twin-screw type was designed. The first disk extruders were developed in the early 1960’s.
Screw extruders (see Figure I) are the type most commonly used in industry. The screw receives the raw material (in granular, powder, strip, or other form) from the feed device and transports it along the housing. The material is compressed (the pressure inside the extruder increases to 15–50 meganewtons per m2[150–500 kg-force per cm2]), heated, plasticized, and homogenized. Depending on the rate of rotation of the screw, extruders may be classified as standard-speed (with peripheral speeds up to 0.5 m/min) and high-speed (up to 7 m/min) types; they may also be classified according to design type as stationary extruders, extruders with a rotating housing, and extruders with a horizontal or vertical screw. Some types have screws that undergo both rotary and reciprocating motion. In order to homogenize the material efficiently, the screws may be provided with supplementary devices, such as teeth, grooves, disks, and lobes. Models with planetary rollers with from four to 12 additional screws rotating around a primary spindle are coming into increasing use.
The operating principle of disk extruders uses the stresses occurring at right angles to the shear stresses in a viscoelastic material. Disk extruders are designed with two disks whose planes are parallel; one disk rotates and the other is fixed. Such an arrangement produces both shear and normal stresses. The fixed disk has a hole in the center, through which the softened material is forced. Disk extruders exhibit better softening and homogenizing capabilities than screw types, but they generate a lower shaping pressure. Consequently, they are used chiefly as mixers and granulators and for the preparation of material to be fed to a screw. Combination extruders with independent drives for a screw and a disk have the advantages of both types.
Because of low productivity, the use of piston extruders is limited chiefly to the manufacture of tubing and shaped articles made of thermosetting plastics.
An extrusion head consists of a heated housing attached to the extruder and a die with an aperture in the shape of a slit constricted at the center (for sheets and films) or an annular channel (for tubes or other products having a circular cross section).
Modern extruders are automated, with a productivity as high as 3–3.5 tons per hr. The proportion of thermoplastic materials processed by extruders varies in different countries, ranging from 30 to 50 percent.
REFERENCESBernhardt, E. C. (comp.). Pererabotka termoplastichnykh materialov. Moscow, 1962. (Translated from English.)
Zavgorodnii, V. K., E. L. Kalinchev, and E. G. Makharinskii. Oborudovanie predpriiatii po pererabotke plastmass. Leningrad, 1972.
Oborudovanie dlia pererabolki plastmass. Moscow, 1976. [Torner, R. V. Teoreticheskie osnovy pererabolki polimerov. Moscow, 1977.
M. L. FRIDMAN