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Melting Lead and Releasing Forms
Name: Haim
Status: Student
Grade: 9-12
Location: Outside U.S.
Country: Israel
Date: April 2007
Question:
I would like to create weight belt
for diving. The main idea is to melt lead into an
aluminium (or iron) mold. However, I do want to put
a metal piece within the aluminium mold in order to
form a hole for the belt (entering the leads
weights). After the melted lead is cold within the
mold, I need to pull-out the metal-piece! I have
heard that I need to lubricate this piece with
graphite before, in order not to be stuck in the
lead. Would this work?
Replies:
Hi Haim,
The short answer to your question is, no, attempting to
lubricate the piece of metal (called a "mold insert" in
Engineering terms) with graphite, would not do much good.
The main problem here is that the lead shrinks as it cools,
and will tend to shrink tightly onto the mold insert making
it very difficult to pull out.
What a designer typically does in such a situation is to make
the mold insert have a so-called "draft angle". That means
that the metal piece you describe (the mold insert), should
be made to have a taper from one end to the other. Typically,
its sides should be "drafted" or tapered at least 2 degrees,
and preferably 5 degrees in the direction you are trying to
pull it out.
To clarify with a simplistic example, imagine what you wanted
to have a round hole in the weight. Instead of using a round
metal rod as a mold insert to make the hole, you would use a
slightly cone-shaped rod whose sides were angled 2 to 5
degrees. To remove it from the finished lead casting, you
only need to tap the narrow end and it will come free and
easily drop out.
Your mold insert used to allow insertion of the belt will
likely be rectangular, but the same principle applies. Taper
all sides by 2 to 5 degrees, and it will be easy to remove.
Note that the exact same principle should be used for the
weight's mold. The mold sides should not be straight, but
should also have a "draft angle" to allow the cast weight to
easily drop out. Use of draft angle when designing both metal
and plastic molds is an extremely important part of mold
design.
I should also point out that iron or steel would be a better
mold material than aluminum. Aluminum expands and contracts
more than lead or iron, so as the mold cools, it shrinks
faster than the lead and tends to shrink tightly to the lead.
If you do use an aluminum mold or mold insert, your draft
angles should be not less than 5 degrees to help reduce the
above problem.
Regards,
Bob Wilson
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