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MATERIAL
PREPARATION
Raw materials of
all kinds have to be converted to what is often called 'stock
materials'. These are usually materials of a given cross section cut
into manageable lengths. This involves the conversion of raw
materials using processes such as rolling, drawing and extrusion to
produce bars, sheets and tubes of various sizes.
Typical cross sections
for the following materials can be found by following the hyperlinks
below:
The conversion of raw
materials into stock materials is known as primary processing. The
main primary processing techniques are explained below:
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| Rolling
Hot rolling is a process whereby
heated material (usually metal) is rolled into long lengths by
squeezing it between heavy rollers. |
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| The
process is usually carried out in stages with the distance between
the rollers being reduced at each stage so that the material becomes
progressively thinner.
Inevitably the metal will cool as it
is rolled and as it cools it becomes more difficult to roll. It is
often necessary to re-heat the material several times to achieve the
required size.
Cold rolling is a similar process to
hot rolling except that the pressure produced by the rollers has to
be much greater because the cold material is much more difficult to
work.
The main advantages of cold rolling
are:
 | A good surface finish |
| A more accurate cross
section |
| A more uniform cross
section |
| Improved strength and
hardness (due to work
hardening) |
Metal sheets with a high surface
finish can be produced with highly polished rollers in a clean air
environment. |
| Extrusion
Extrusion is the process whereby
stock materials are formed by forcing them through a die.
The shape of the material section
produced is exactly the same as the shape of the hole in the die.
|
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| Most
materials are heated before they are extruded although ductile
metals such as aluminium are sometimes extruded cold. |
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Some Common Die
Shapes
' L'
Section
' I '
Section
' T '
Section
Round
Tube
Square Tube |
| Drawing |
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| Drawing
is a similar process to extrusion except that the material is pulled
or 'drawn' through a series of dies.
Drawing is often used to |
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produce rods, wires or
tubes. Very fine wires can be produced by drawing through a series
of dies with successively smaller openings.
The amount by which the
diameter can be reduced at each stage is limited by the force which
can be used to pull the material through the die and the ductility
of the material being used. As with most cold working processes the
material will work harden and may need to be annealed several times
before then required size and section is produced. |
