Extrusion

EXTRUSIONCh. 5ExtrusionA compression forming process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape.Pros:variety of sections possible (hot extrusion)grain structure and strength enhancement (cold)close tolerance (cold)no material wastage.EXTRUSIONS

STANDARD EXTRUSIONS

Extrusions

Figure 15.2 Extrusions, and examples of products made by sectioning off extrusions. Types of ExtrusionDirect ExtrusionThe ram forces the work billet metal to move forward to pass through the die opening.Indirect ExtrusionThe die is mounted to the ram rather than at the opposite end of the extruder container housing.

Direct Extrusion

Figure 15.1 Schematic illustration of the direct extrusion process.Direct ExtrusionFriction increases the extrusion force.Hollow section is formed using a mandrel.

Indirect Extrusion

Figure 15.3 Types of extrusion: (a) indirect; (b) hydrostatic; (c) lateral.Indirect ExtrusionMetal is forced to flow through the die in an opposite direction to the rams motion.Lower extrusion force as the work billet metal is not moving relative to the container wall.

Hydrostatic ExtrusionHydrostatic ExtrusionUsing hydrostatic system to reduce the friction and lower the power requirement.Sealing is the major problem.

Ram ForceVariation of Ram Force with ram stroke and die angle.

Lateral ExtrusionExtrusion-Die Configurations

(a)(b)

(c)Figure 15.8 Typical extrusion-die configurations: (a) die for nonferrous metals; (b) die for ferrous metals; (c) die for T-shaped extrusion, made of hot-work die steel and used with molten glass as a lubricant. Hydraulic-Extrusion Press

Figure 15.17 General view of a 9-MN (1000-ton) hydraulic-extrusion press. Extrusion Equipment

EXTRUSION

EXTRUSION

Cross-Sections to be Extruded

Figure 15.10 Poor and good examples of cross-sections to be extruded. Note the importance of eliminating sharp corners and of keeping section thicknesses uniform. Source: J. G. Bralla (ed.); Handbook of Product Design for Manufacturing. New York: McGraw-Hill Publishing Company, 1986. Used with permission.Components for Extruding Hollow Shapes

Figure 15.9 (a) An extruded 6063-T6 aluminum ladder lock for aluminum extension ladders. This part is 8 mm (5/16 in.) thick and is sawed from the extrusion (see Fig. 15.2). (b)-(d) Components of various dies for extruding intricate hollow shapes. Source: for (b)-(d): K. Laue and H. Stenger, Extrusion--Processes, Machinery, Tooling. American Society for Metals, Metals Park, Ohio, 1981. Used with permission.Extrusion ProcessesHot extrusionKeeping the processing temperature to above the re-crystalline temperature. Reducing the ram force, increasing the ram speed, and reduction of grain flow characteristics. Controlling the cooling is a problem. Glass may be used as a lubricant.Cold extrusionOften used to produce discrete parts. Increase strength due to strain hardening, close tolerances, improved surface finish, absence of oxide layer and high production rates.Extrusion Temperature Ranges for Various Metals

Extrusion Defectsa)Centre-burst: internal crack due to excessive tensile stress at the centre possibly because of high die angle, low extrusion ratio.b) Piping: sink hole at the end of billet under direct extrusion.c) Surface cracking: High part temperature due to low extrusion speed and high strain rates.

Process Variables in Direct Extrusion

Figure 15.4 Process variables in direct extrusion. The die angle, reduction in cross-section, extrusion speed, billet temperature, and lubrication all affect the extrusion pressure.Extrusion AnalysisTaking into account friction, where a =0.8 and b =1.2 to 1.5.

Extrusion ratio,

Assuming all sections are circular, ideal deformation, no friction,no redundant work:

Ram pressure

Extrusion AnalysisFor direct extrusion, additional pressure, pf, required by the extruder to overcome the wall friction is related as follows:

For the worst case that the friction shear stress at the wall equals to the shear yield strength of the work metal:

The additional pressure:

The total ram pressure:

The power required:

Extrusion Dies For the case of non-circular extruded section, a shape factor has to be introduced:

where Kx = shape factorCx = perimeter of the non-circular extruded sectionCo = perimeter of a circle that has the same cross-sectional area as the extruded section.For direct extrusion, the extrusion force

Impact ExtrusionImpact extrusion is performed at higher speeds and shorter strokes than conventional extrusion.It is for making discrete parts.For making thin wall-thickness items by permitting large deformation at high speed.

Figure 15.14 Schematic illustration of the impact-extrusion process. The extruded parts are stripped by the use of a stripper plate, because they tend to stick to the punch.Impact ExtrusionImpact Extrusion

ForwardbackwardcombinationExamples of Cold Extrusion

Figure 15.11 Two examples of cold extrusion. Thin arrows indicate the direction of metal flow during extrusion.Examples of Impact Extrusion

Figure 15.15 (a) Two examples of products made by impact extrusion. (b) and (c) Impact extrusion of a collapsible tube by the Hooker process.Cold Extruded Spark Plug

Figure 15.12 Production steps for a cold extruded spark plug.

Figure 15.13 A cross-section of the metal part in Fig. 15.12, showing the grain flow pattern. Factors Influencing the ForcesFrictionMaterial PropertiesReduction In AreaSpeedTemperatureGeometry Of The DieWIRE DRAWING

WIRE DRAWINGWIRE DRAWING

WIRE DRAWINGProcess Variables in Wire Drawing

Figure 15.18 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.Draw DiesApproach angle about 6 to 20Back relief angle about 30

Die for Round Drawing

Figure 15.20 Terminology of a typical die used for drawing round rod or wire.

Figure 15.21 Tungsten- carbide die insert in a steel casing. Diamond dies, used in drawing thin wire, are encased in a similar manner.Die Materials OverviewTungsten Carbide: Lowest cost, shock resistance, ease of production, large sizes available. Lower life expectancy. Natural Diamonds:Wear resistance, gives excellent wire surface, high thermal conductivity, longer life expectancy Susceptible to fractures from shock or wear, limited availability in required high quality and quantity, constantly escalating price. Synthetic Single Crystal:Consistently uniform material, gives excellent wire surface, high thermal conductivity, predictable wear schedule, uniform wear pattern gives longer life expectancy. Larger size ranges are still costly at this time. Polycrystalline Diamond:Excels in life expectancy, wear resistance of diamond, shock resistance of carbide, high availability, cost effectiveness Higher drawing force, smaller fines requires more filtration, may be damaged by temperatures above 700C, wire surface condition less than from natural diamond.

Examples of Tube-Drawing Operations

Figure 15.19 Examples of tube-drawing operations, with and without an internal mandrel. Note that a variety of diameters and wall thicknesses can be produced from the same initial tube stock (which has been made by other processes).

Drawing EquipmentGood dimensional controlGood surface finishImproved mechanical propertieseconomic for mass production

Cold Drawing

Figure 15.23 Cold drawing of an extruded channel on a draw bench, to reduce its cross-section. Individual lengths of straight rod or of cross-sections are drawn by this method. Source: Courtesy of The Babcock and Wilcox Company, Tubular Products Division.

Figure 15.24 Two views of a multistage wire-drawing machine that is typically used in the making of copper wire for electrical wiring. Source: H. Auerswald.Multistage Wire-DrawingRoll StraighteningFigure 15.22 Schematic illustration of roll straightening of a drawn round rod (see also Fig. 13.7).

Wire and Bar DrawingReducing the cross section of a bar, rod or wire by pulling it through a die.Bar drawing is generally in a batch mode while the wire drawing is in general in a continuous mode.

pd Drawing die(deformation zone)Stresses and drawing pressure variation in deformation zone.Effective strain variation in deformation zone. Drawing die angleFig. 5.2 Stress and strain development in wire and rod drawing process 2 3

1 Af Ao =ln(Ao/Af) s pd s pd=K/(n+1)e n+1 s* s=Ke n

e* e

Fig. 5.4 Stresses and drawing pressure for maximum reduction of area.Mechanics of DrawingArea reduction in drawing ; Draft

No friction and true strain

The ideal stress Taking into account friction and die angle,

where , ,

The drawing force

Maximum Reduction Assume perfectly plastic material (n=0), no friction, no redundant work,

Then

The maximum possible area ratio

The maximum possible reduction

(C

Lead 200250

Aluminum and its alloys 375475

Copper and its alloys 650975

Steels 8751300

Refractory alloys 9752200

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