Department of Mechanical Engineering

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ME 321

Manufacturing

Process I

Instructor : Prof. Dr. Omar Badran

Prepared by:

Dr. Naseer Ahmed

Department of Mechanical Engineering

College of Engineering, Taibah University

Department of Mechanical Engineering

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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

Two principal types of extrusion are

Direct Extrusion

Indirect extrusion

Another classification is by working temperature

Cold extrusion

Warm extrusion

Hot extrusion

Also

Continuous process

Discrete process

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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

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Indirect Extrusion

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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. 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.

Department of Mechanical Engineering

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Continuous vs Discrete Processing

Continuous processing is approached by producing very long sections in one cycle, but these operations are ultimately limited

by the size of the billet that can be loaded into the extrusion

container. These processes are more accurately described as

semi-continuous operations.

In Discrete extrusion operation a single part is produced in each extrusion cycle. Impact extrusion is an example of the discrete

processing case.

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Analysis of Extrusion

Taking into account friction,

where a =0.8 and b =1.2 to 1.5.

The value of a and b tends to increase with increasing die angle

Extrusion ratio,

f

ox

A

Ar

Assuming all sections

are circular, ideal

deformation, no friction:

xrln

Ram pressure, Johnsons formulae xf rYp ln

)ln( xf rbaYp

xx rba ln

Department of Mechanical Engineering

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Analysis of Extrusion For direct extrusion, The ram pressure:

Ram force in indirect or direct extrusion

Power required to carry out the extrusion operation is

F = ram force, N; v = ram velocity, m/s; P = power, J/s

o

xfD

LYp

2

FvP

opAF

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Example

A billet 75 mm long and 25 mm in diameter is to be extruded in a direct extrusion operation with extrusion ratio rx = 4.0. The

extrudate has a round cross section. The die angle (half-angle) =

90o. The work metal has a strength coefficient = 415 MPa, and

strain hardening exponent = 0.18. Use the Johnson formula with

a = 0.8 and b = 1.5 to estimate extrusion strain. Determine the

pressure applied to the end of the billet as the ram moves

forward.

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A complex extruded cross section for a heat sink (photo

courtesy of Aluminum Company of America)

Complex Cross Section

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Extrusion Presses

Either horizontal or vertical

Horizontal more common

Extrusion presses - usually hydraulically driven, which is especially suited to semi-continuous direct extrusion of long

sections

Mechanical drives - often used for cold extrusion of individual parts

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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 and

impurities in the work metal.

b) Piping: sink hole at the end of billet under direct extrusion.

c) Surface cracking: High part temperature due to high

extrusion speed and high strain rates.