posted 01-20-2001 02:53 PM
If you are going to make an extremely thin rubber shell with liquid latex, you can do this in any type mold by painting in layers, but for a thicker item you'll want to do slush casting. To make a rubber hand with a rubber that requires vulcanising/heating (Adrub or vinyl-Plastisol) you need a sturdy mold, so go with Ultracal 30. A silicone rubber casting can be made in almost any type of mold, it sticks to practically nothing.I'll assume you want to slush cast liquid latex:
Slush Casting
Latex slush casts best in a mold made with highly absorbative material. This means white art plaster or hydrocal A, either one well dried out before casting in it. Forget Ultracals or dental stones and don't add acrylic fortifier to the mold.
The simplest approach is to do an alginate mold of the hand (backed up with a mother mold of plaster bandages or other rigid material) and make a flexible cast in it. Cast more alginate or a low temperature casting wax (flexwax or microcrystalline wax) in the alginate mold of the hand.
This can be used to make a one piece plaster mold or hydrocal A mold. Melt out the wax (or slice it up carefully if using alginate) to remove the positive.
Dry out the mold by gently baking it for an hour or so on very low heat (be careful, molding plaster is weak and can collapse if over baked). Let it cool slowly to avoid cracking the mold.
Slush cast the hand by filling the mold with liquid latex and waiting for a thick skin to build up on the mold walls as the water & ammonia are absorbed into the mold walls. This is called dwell time" and varys from latex to latex, but 30 - 45 minutes isn't unusual. Pour out nost of the latex (still liquid) and let the thick "slush" layer dry. Warm, moving air (hair drier) speeds this considerably.
This hollow rubber obect can be trimmed and painted and filled with whatever you like or find necessary.
You can make a two piece mold, but then you have to deal with removing mold seams on the cast piece.
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Jeff F - Moderator
Magic and FX
Amazing the Masses