2. Introduction
Extrusion – It is a process where a billet is forced
through a die.
• Parts have constant cross-section
• Typical Products of Extrusion – Sliding Doors,
tubing having various cross-sections, structural and
architectural shapes and door and window frames.
.
4. The Extrusion Process
Types of Extrusion :
1-Direct Extrusion
Billet is placed in a chamber and forced through a die opening by a hydraulically-driven
ram or pressing stem.
2-Indirect Extrusion
Die moves towards the billet.
3-Hydrostatic Extrusion
The billet is smaller in diameter that the chamber, which is filled with a fluid, and the
pressure is transmitted to the billet by a ram.
EExxttrruussiioonn RRaattiioo = Ao/Af
Ao – cross-sectional area of the billet
Af - cross-sectional area of extruded product
6. Process Variables in Direct Extrusion
Fig : Process variables in direct
extrusion. The die angle,
reduction in cross-section,
extrusion speed, billet
temperature, and lubrication
all affect the extrusion
pressure.
Fig : Method of determining the
circumscribing-circle
diameter (CCD) of an
extruded cross-section.
7. Types of Metal flow in extruding with square dies.
Fig : Types of metal flow in extruding with square dies. (a) Flow pattern obtained at low friction, or in indirect
extrusion. (b) Pattern obtained with high friction at the billet-chamber interfaces, (c) Pattern obtained at
high friction, or with cooling of the outer regions of the billet in the chamber. This type of pattern, observed
in metals whose strength increases rapidly with decreasing temperature, leads to defect known as pipe, or
extrusion defect.
8. Hot Extrusion
• Extrusion is carried out at elevated temperatures – for metals and
alloys that do not have sufficient ductility at room temperature, or
in order to reduce the forces required.
9. Advantages of Hot Extrusion
• This reduces strength and increase ductility
of the metal
• Complex shapes are achieved by this
method
• Reduction of ram force.
• Increased ram speed
• Reduction of grain flow
10. Cold Extrusion
Combination of operations, such as direct and indirect extrusion and forging.
Advantages :
1-Improved mechanical properties
2-Good control of dimensional tolerances
3-Improved surface finish
4-Elimination of the need for billet heating;
Fig : Two examples of cold
extrusion. Thin arrows
indicate the direction
of metal flow during
extrusion.
11. Advantages of Cold Extrusion
• Increased strength due to strain hardening
• Close tolerance
• Improved surface finish
• Absence of oxide layer
• High production rate
12. Impact Extrusion
• Similar to indirect extrusion
• Punch descends rapidly on the blank, which is extruded backward
Fig : 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.
13. Examples of Impact Extrusion
Fig : (a) Two examples of products made by impact extrusion. These parts may also be made by casting, by forging,
or by machining; the choice of process depends on the dimensions and the materials involved and on the
properties desires. Economic considerations are also important in final process selection. (b) and (c) Impact
extrusion of a collapsible tube by the Hooker process.
14. Hydrostatic Extrusion
• The pressure required for extrusion is supplied through and incompressible fluid
medium surrounding the billet
• Usually carried at room temperature, typically using vegetable oils as the fluid
• Brittle materials are extruded generally by this method
• It increases ductility of the material
• It has complex nature of the tooling
Fig : General view of a 9-
MN (1000-ton)
hydraulic-extrusion press.
15. Extrusion Defects
1. Surface cracking.
2. Pipe.
3. Internal Cracking
Fig : (a) Chevron cracking (central burst) in extruded round steel bars. Unless the products are inspected, such internal
defects may remain undetected, and later cause failure of the part in service. This defect can also develop in the
drawing of rod, of wire, and of tubes. (b) Schematic illustration of rigid and plastic zones in extrusion. The
tendency toward chevron cracking increases if the two plastic zones do not meet. Note that the plastic zone can be
made larger either by decreasing the die angel or by increasing the reduction in cross-section (or both).
