MECH152-L20-1 (1.0) - 1
Extrusion
MECH152-L20-1 (1.0) - 2
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 Extrusion
The ram forces the work billet metal to move
forward to pass through the die opening.
Indirect Extrusion
The 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 Extrusion
Metal 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 Processes
Hot 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 extrusion
Often 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
Extrusion ratio, rx 
Ao
Af
Assuming all sections
are circular, ideal
deformation, no friction,
no redundant work:
  ln rx
Ram pressure
p  Yf ln rx
Taking into account friction, p  Y f (a  b ln rx )
where a =0.8 and b =1.2 to 1.5.
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Extrusion Analysis
For direct extrusion, additional pressure, pf, required by the
extruder to overcome the wall friction is related as follows:
p f Do2
4
 pcDo L (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:
pc  Ys  Yf
(2)
Subs. (2) into (1), the additional pressure:
The total ram pressure: p  Y f   x  2L 
The power required: P  Fv

Do 
p f  Yf
2L
Do
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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:
C 
K x  0.98  0.02 x 
 Cc 
2.25
where Kx = shape factor
Cx = perimeter of the non-circular extruded section
Co = perimeter of a circle that has the same crosssectional area as the extruded section.
For direct extrusion, the extrusion force

2L 

p  K xY f   x 
Do 

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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 Drawing
Area reduction in drawing r 
Ao  Af
Ao
No friction and true strain   ln
; Draft d  Do  D f
Ao
1
 ln
Af
1 r
The ideal stress   Y f   Y f ln Ao
Af


Do  D f
Taking into account friction and die angle,  d  Y f 1 
where   0.88  0.12
Do  D f
D
Lc 
D

,
,
2 sin 
2
Lc
The drawing force F  Af  d  Af Y f 1 

Ao
 

ln

tan  
Af
A
 
 ln o
tan  
Af
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Maximum Reduction
Assume perfectly plastic material (n=0), no friction, no
redundant work,
Ao
Ao
1
 d  Y f ln
 Y ln
 Y ln
Y
Af
Af
1 r
Then ln Ao Af  ln 1 1  r  1
Ao A f  1 1  r  e
 max  1.0
The maximum possible area ratio Ao A f  e  2.7183
The maximum possible reduction rmax  e  1 e  0.632
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Drawing Equipment
Good dimensional
control
Good surface finish
Improved mechanical
properties
economic for mass
production
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Draw Dies
Approach angle about 6 to 20
Back relief angle about 30
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Tube Drawing
No mandrel
Fixed mandrel
Floating mandrel