It depends on the neck in relation to the diameter of the rest of the
piece. It's always easier to stretch material than compress it so when
the neck is fairly large relative to the body (like a mason jar) they
tend to use an expanding die on the inside. When the neck is small or
needs to be formed thicker and with threads or similar, it is
compressed. Must be very controlled and the process carefully designed.
Generally, it's done as the last step of the process. Sometimes there
is a final overall sizing step.
Note that to draw the initial shell probably takes several press hits in
different dies. If you look on the bottom of an aluminum can (like
those fuel bottles for backpacking) you can see that they have been
drawn about 3 times to get the final shape.
There are other ways too, depending on the bottle shape. There are even
ways to blow mold aluminum similar to the way plastic bottles are done.
Anyway, if you give more details as to your specific question or the
dimensions that have you wondering about the process, I can probably
scan a few pics from a pressworking book that may help.
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