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Extrusion

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Extrusion

A 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.

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Types of Extrusion

Direct 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.

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Direct Extrusion

Friction increases the extrusion force.

Hollow section is formed using a mandrel.

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Indirect ExtrusionMetal is forced to flow through the die in an opposite direction to the ram’s motion.

Lower extrusion force as the work billet metal is not moving relative to the container wall.

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Extrusion ProcessesHot extrusion

Keeping 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.

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Extrusion Analysis

Taking into account friction,

where a =0.8 and b =1.2 to 1.5.

Extrusion ratio,f

ox A

Ar

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

xrlnRam pressure

xf rYp ln

)ln( xf rbaYp

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Extrusion AnalysisFor direct extrusion, additional pressure, pf, required by the

extruder to overcome the wall friction is related as follows:

(1) [equilibrium equation]

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

(2)

Subs. (2) into (1), the additional pressure:

The total ram pressure:

The power required:

LDpDp

ocof

4

2

fsc YYp

off D

LYp

2

oxf D

LYp

2

FvP

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Ram Force

Variation of Ram Force with ram stroke and die angle.

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Extrusion Dies For the case of non-circular extruded section, a shape factor

has to be introduced:

where Kx = shape factor

Cx = perimeter of the non-circular extruded section

Co = perimeter of a circle that has the same cross-sectional area as the extruded

section.

For direct extrusion, the extrusion force

25.2

02.098.0

c

xx C

CK

oxfx D

LYKp

2

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Impact Extrusion

Forward backward

combination

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Impact Extrusion

Impact 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.

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Hydrostatic Extrusion

Using hydrostatic system to reduce the friction and lower the power requirement.

Sealing is the major problem.

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Extrusion Defects

a) 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.

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Wire and Bar Drawing

Reducing 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.

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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

o

fo

A

AAr

fo DDd

rA

A

f

o

1

1lnln

f

off A

AYY ln

f

ofd A

AY ln

tan1

cL

D12.088.0

2fo DD

D

sin2fo

c

DDL

f

offdf A

AYAAF ln

tan1

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

redundant work,

Then

The maximum possible area ratio

The maximum possible reduction

Yr

YA

AY

A

AY

f

o

f

ofd

1

1lnlnln

erAA

rAA

fo

fo

11

111lnln

0.1max

7183.2eAA fo

632.01max eer

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Drawing EquipmentGood dimensional control

Good surface finish

Improved mechanical properties

economic for mass production

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Draw Dies

Approach angle about 6 to 20Back relief angle about 30

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Tube Drawing

No mandrel

Fixed mandrel

Floating mandrel