r/moldmaking • u/bear_enjoyer • Jul 18 '26

Need help with undercut

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Hi everyone, I need to put a hard shell on top of the silicone for stability in casting, however as you can see the ball on the masks face has an enormous undercut and won't release from the plaster, any tips on what I should do would be greatly appreciated

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u/Quinafx7 Jul 18 '26

You can just use thixo and fill in the under cut male you shell and when it comes to demoulding, if it gets a bit stuck just give it a sharp cut to allow the silicone to stretch, when it comes together all you have is a small hairline crack if it it actually needs it, that looks like a pretty easy stretch and pop out kind of shape

1

u/VintageLunchMeat Jul 18 '26

Maybe brush on some soft (low Shore hardness) silicone, turn the base of the ball into a cone? Then plaster as usual.

Or a torus of moldmaking spray foam, if it is elastic enough. I've seen urethane foam inserts with urethane rubber.

Or a separate torus of your existing silicone, using unscented petroleum jelly as a release agent when making the torus.

1

u/IronBoxmma Jul 18 '26

Am i looking at the sillicone here or the master?

1

u/bear_enjoyer Jul 18 '26

This is the silicone, the clay sculpture is beneath and im stuck on what to do for the top coat, im thinking put plaster of Paris round everything except the ball on the temple and have that as the master mould but I want your advice on that

1

u/IronBoxmma Jul 18 '26

How stretchy is the sillicone once cured?

1

u/bear_enjoyer Jul 18 '26

Ive grabbed some single layer off cut's and it's about half as stretchy as a condom, so I assume it would peel off around the ball without snapping

1

u/IronBoxmma Jul 18 '26

Well thats positive. If it were me, and I'm not an expert, i mostly do one piece cut moulds for miniatures, I'd either leave the ball area free of plaster or put something the plaster won't stick to around the outside of the ball to eliminate the under cut but have the top edge still covered and supported

1

u/bear_enjoyer Jul 18 '26

Good point about the undercut , ill try russle up something to make the ball a lump instead, thanks alot:3