Aluminium Design and Construction John Dwight

Figure 2.2 Typical extrusion die.
the thin parts, so that a section such as the one shown would come out
in a curve if nothing were done. To counter this effect, the die designer
retards the flow of metal in the thick regions by increasing the depth
of ‘land’ (dimension x). Even so, a section never comes off the press
completely straight and subsequent stretching is always needed.
The performance of a die can be improved if any re-entrant corners
in the aperture (outside corners on the section) are slightly radiused,
even with a radius of only 0.3 mm. It is bad practice to call for absolutely
sharp corners unless these are essential. They increase the risk of die
failure and reduce the permitted extrusion speed.
The pressure acting on the face of a die during extrusion is very
high, possibly approaching 700 N/mm 2 . With sections such as those
shown in Figure 2.3, there is the possibility that the die will break along
line Y due to the pressure acting on region X. This tendency depends
on the aspect ratio (a) of the region X, defined by:
(2.3a)
(2.3b)
where c, d are as defined in the figure and A is the area of region X.
Under the most favourable conditions, i.e. for a section in 6063 alloy (or
Figure 2.3 Re-entrants in extruded sections.
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