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Name: Haim
Status: Student
Grade: 9-12
Location: Outside U.S.
Country: Israel
Date: April 2007

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?

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.


Bob Wilson

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