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Christopher W. Jowett, Gary Hay and Nick Parson, Alcan International Limited


ABSTRACT --- The way the billet upsets in the container can have a significant
impact on the extrusion process. It can determine if air is trapped during the upset,
and where. It can affect the way metal flows through dies at the beginning of the
extrusion cycle. It can also influence the extent to which billet skin is prematurely fed
into the extrusion. The mode of upset is not well understood. Mathematical modeling
can give an idealized picture and logic can suggest how billets might upset, but in
the end there is no real alternative to observing the actual process. This paper
describes a technique that has allowed us to watch the way the billet upsets under
different real extrusion conditions. Results obtained allow us to consider ways to
reduce air entrapment and hence blistering. Variables considered in the
experimental tests included extrusion alloy (both 3xxx and 6xxx), the clearance
between the billet and container, the billet/container temperature differential, both
billet and container temperature tapers as well as burp and upset pressure.

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