Extrusion and Drawing

Chapter (8) Extrusion and drawing 8.1

Introduction Extrus rusion

and

draw rawing

have ave

numer umerou ouss

appli plications

in

the

manufacture of continuous as well as discrete products from a wide variety variety of metals metals and alloys. alloys. In extrusi extrusion, on, a cylind cylindrica ricall billet billet is forced forced through a die (Fig. 8.1) in a manner similar to squeezing toothpaste from a tube or extruding Play-Doh® in various cross-sections in a toy press. A wide wide varie variety ty of solid solid or holl hollow ow cross cross-se -sect ctio ions ns may may be produc produced ed by extrusion, which essentially are semifinished parts. A characteristic of extrusion (from the Latin extntdere, meaning "to force out"} is that large deform deformati ations ons can take take place place withou withoutt fractu fracture, re, becaus becausee the materi material al is under high triaxial compression during extrusion. Since the die geometry remains unchanged throughout the operation, extruded products typically have a constant cross-sect cross-section. ion. Lead pipes pipes were made by extrusion extrusion in the 1700s. Plastics are extruded extensively.. Typical products made by extrusion are railings for sliding doors, window frames, frames, tubing tubing having having various various cross-se cross-secti ctions, ons, alumin aluminum um ladders ladders,, and numerous structural and architectural shapes. Extrusions Extrusions can be cut into desired lengths, which then become become discrete discrete parts, parts, such as brackets, brackets, gears, and coat hangers (Fig. 8.2). Commonly

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FIGURE 8.1

FIGURE 8.2

Schematic illustration of the direct-extrusion process.

Extrusions and examples of products made by sectioning off extrusions. Source: Courtesy of Plymouth Extruded Shapes.

extruded extruded materia materials ls are aluminu aluminum, m, copper, copper, steel, steel, magnesiu magnesium, m, and lead; lead; other other metal metalss and alloy alloyss also also can can be extrud extruded, ed, with with vario various us levels levels of difficulty.





economical for large production runs as well as for short ones. Tool costs generally are are low, particularly particularly for producing simple, solid cross-sections. cross -sections. Depending Depending on the ductility ductility of the material, extrusion extrusion may be carried out at room or elevated temperatures. Extrusion at room temperature often is combined with forging operations, in which case it generally is known known as cold cold extru extrusi sion. on. It has has nume numero rous us impo import rtan antt appl applic icat atio ions ns,, incl ncludin ding

fastene teners rs

and and

compo ompone nent ntss

for for

auto automo mobi bile les, s,

bicy bicycl cles es,,

motorcycles, heavy machinery, and transportation equipment. Drawing is an operation developed between 1000 and 1500 A.D., in which which the cross-section of solid rod, wire, w ire, or tubing is reduced or changed in shape by pulling it through a die. Drawn rods are used for shafts, spindles, spindles, and small pistons pistons and as the raw material for fasteners (such as rivets, bolts, and screws). In addition to round rods, rods, various profiles profiles also can be drawn. The term drawing also also is used used to refe referr to makin making g cupcupshaped parts by sheet-metal forming operations. The The dist distin inct ctio ion n betw betwee een n the the term termss rod rod and and wire wire is some somewh what at arbitrary, arbitrary, rod being being large largerr in crosscross-sec sectio tion n than than wire. wire. In indust industry, ry, wire wire generally is defined as a rod that has been drawn through a die at least once. Wire drawing involves smaller diameters than rod drawing with sizes down to 0.01 mm (0.0005 in.) for magnet wire and even smaller for use in very low-current fuses.

8.2 The Extrusion Process There are three basic types of extrusion. In the most common process





ram (pressing stem or punch), as shown m Fig. 8.1. The die opening may be round, or it may have various shapes, depending on the extruded shape desired. The function of the dummy block Is to protect the tip of the pres pressi sing ng stem stem (punc (punch), h), part partic icul ularl arly y in hot hot extr extrusi usion on.. Othe Otherr type typess of extrusion are indirect, hydrostatic, and impact extrusion. In indi indirec rectt extrus extrusio ion n (also (also calle called d revers reverse, e, inve invert rted ed,, or back backwa ward rd extrus extrusio ion), n), the die moves toward toward the unextrude unextruded d billet billet (Fig. (Fig. 8.3a). 8.3a). In hydrostatic extrusion (Fig. 8.3b), the billet is smaller in diameter than the chamber (which is filled with a fluid), and the pressure is transmitted to the billet by a ram. Unlike in direct extrusion, there is no friction to overcome along the container walls because the billet is stationary with respect to the container. A less common type of extrusion is lateral lateral (or

side) extrusion (Fig. 8.3c).

FIGURE 8.3

8.3

Types of extrusion: (a) indirect; (b) hydrostatic; (c) lateral.

Hot Extrusion For metals and alloys that do not have sufficient ductility at room

temp tempera eratu ture re (or (or in order to reduce reduce the forces forces require required), d), extrusi extrusion on is carried out at elevated temperatures (Table 8.1). As in all other hotworking operations, hot extrusion has special requirements because of the high operating temperatures. For example, die wear can be excessive,





the the die die can can resul resultt in highl highly y nonuni nonunifo form rm deform deformat atio ion n (Fig. (Fig. 8.3c 8.3c). ). To reduce cooling of the billet and to prolong die life, extrusion dies may be preheated, as is done in hot-forging operations. Because the billet is hot, it develops an oxide film unless it is heated in an inert-atmosphere furnace. This film can be abrasive, and it can affect the flow pattern of the material. It also results in an extruded pro produ duct ct that that may may be unac unacce cept ptab able le when when a good good surf surfac acee fini finish sh is important. In order to avoid the formation of oxide films on the hot extruded product, prod uct, the dummy bloc k placed ahead a head of the ram (Fig. (Fig. 8.1) 8.1) is made a little smaller in diameter than the container. As a result, a thin shell (skull) consisting mainly of the outer oxidized layer of the billet is left in the container. The skull is removed remov ed later from the chamber. TABLE 8.1 Typical Extrusion Temperature Ranges for Various Metals and Alloys

°C

Lead

200-250

Aluminum and its alloys

375-475

Copper and its alloys

650-975

Steels

8 75 - 1 3 0 0

Refractory alloys

9 75- 22 00

8.4 Cold Extrusion Developed in the 1940s, cold extrusion is a general term that often denotes a combination of operations, such as direct and indirect extrusion





FIGURE 8,4

Two examples of cold extrusion. Thin arrows indicate die direction of metal flow during extrusion.

Cold Cold extr extrus usio ion n is used used wide widely ly for for compo componen nents ts in auto automo mobi bile les, s, motorc motorcycl ycles, es, bicycl bicycles, es, and applian appliances ces and in transp transporta ortatio tion n and farm farm equipment. This process uses slugs cut from cold-finished or hot-rolled bars, wire wire,, or plate plates. s. Slug Slugss that that are are less less than than abou aboutt 40 mm (1.5 (1.5 in.) in.) in diameter are sheared (cropped), and if necessary, their ends are squared off off by proce process sses es such such as upset upsetti ting ng machi machinin ning, g, or grindi grinding. ng. Larger Larger-diamet diameter er slugs slugs are machin machined ed from from bars bars into into specif specific ic lengt lengths hs.. Cold Cold-extruded extruded parts weighing as much as 45 kg (100 Ib) and having lengths of up to 2 m (80 in.) can be made, although most parts weigh much less. Powder-metal slugs (preforms) also may be cold extruded.

8.5 The Drawing Process In drawing, the the cross cross-se -sect ctio ion n of a long long rod rod or wire wire typi typica call lly y is reduced or changed by pulling pulling (hence the term drawing) drawing) it through through a die called a draw die (Fig. 8.5). Thus, the differenc differencee between between drawing and







wide wide range range of appl applic icat atio ions ns,, incl includi uding ng rods rods for for shaf shafts ts for for powe powerr and and motion transmissions, machine and structural components, and as blanks for bolts bolts and rivets, rivets, electrical electrical wiring, wiring, cables, cables, tension-loa tension-loaded ded structural structural members, members, welding welding electrodes, electrodes, springs springs,, paper paper clips, clips, spokes spokes for bicycl bicyclee wheels, and stringed musical instruments. The major processing variables in drawing are similar to those in extrusion-t extrusion-that hat is, reducti reduction on in cross-se cross-secti ctional onal area, area, die angle, angle, fricti friction on along along the die-workpi die-workpiece ece interfaces, interfaces, and drawing speed. The die angle influences the drawing force and the quality of the drawn product.

FIGURE 8.5 Process variables in wire drawing. The die angle, the reduction in cross-sectional area per pass, the speed of drawing, the temperature, and the lubrication all affect the drawing force, F.





variety of diameters and wall thicknesses can be produced from the same initial tube stock (which has been made by other processes).

As can be seen from these equations, the drawing force increases as reduction reduction increases. However, there has to be a limit to the magnitude of the force, because when the tensile stress reaches the yield stress of the metal being drawn, the work-piece will simply yield and, eventually, break. It can be shown that, ideally and without friction, the maximum reducti reduction on in cross-se cross-secti ctiona onall area per pass is 63%. Thus, for example, a 10-mm diameter rod can be reduced (at most) to a diameter of 6.1 mm in one pass without failure. It can be shown that, for a certain reduction in diameter and a certain friction-al condition, there is an optimum optimum die angle at which the drawing drawing force is a minimum. This does not mean that the process should be carried out at this optimum angle, because there are other product quality considerations. Drawi Drawing ng of other other shapes shapes.. Various Various solid solid cross-se cross-secti ctions ons can be

produced produced by drawing drawing throu through gh dies dies with with diff differe erent nt profi profile les. s. Prope Properr die design design and the proper selection selection of reduction reduction sequence per pass require considerable experience to ensure proper material flow in the die, reduce internal or external defects, and improve surface quality.





is the fundamental deformation mechanism in ironing, used extensively in making aluminum beverage cans,

Extrusion and Drawing

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