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Can you use a Plaster mold as a Blow mold? Answered

I wanted to make a hollow plastic shape & not able to afford a metal blow mold. Can I use a plaster mold with wood reinforcement as a substitute for the metal one?

The Idea is to heat up a plastic bottle that can fit in the mold cavity & slowly blow it up with an air compressor.

You think this might work, or do you think I may need more reinforcement in the plaster?


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1 year ago

I would go like this:
1. Make a box that is big enough from some leftover wood.
2. Drill lots of holes into the side walls and if you need to expand down then at the bottom as well.
5-8mm holes will do fine.
3. Tape some paper towels or similar over the inside of the walls.
4. Mix your plaster and get the positive for the mold done before.
If it round as in your pic you want to make two molds, one for the bottom half, one for the top half and join them once the plaster is set.

You need to wait about a week until the plaster is fully cured and dry.
Plaster lets air thrugh quite easy but you still need to be careful with th pressure as you don't want to blow up more than what air can escape through the plaster.
If you need a smoother finnish on the outside of the finnished product try dusting your mold with corn starch.