Moulding for rotocasting - advice needed

[Inlite]

Apologies in advance that this isn't for a prop but this forum seems to be the best place for advice on making things.

I am making a large "space whale" which is 3d printed and now I am making a silicone mould of it. I plan to make a kind of rotocaster for it in the end.
So I am currently layering up silicone on the body of the whale and when that is a decent thickness ( 1cm/ half inch ?) then I want to make a 2-part fibreglass shell to support it for the casting.

The question is, I can foresee the silicone sagging inside the fibreglass shell. Am I on the wrong track? or is there a way to keep it against the support shell so I can roto mould it?



This is not the first thing I have cast in resin but the largest and most awkward so any advice would be much appreciated.

 

[JPH]

I would cast in Sorta clear, 2 sections. Then you can use fiberglass or plaster o paris or FreeForm air for outer, hard, crunchy shell.

Sorta Clear is pretty firm when dries. I used it to blind rotocast stuff. Since it is see thru, you know your rotocasting is covering everything

 

[Inlite]

Thankyou for the advice! I wish I had posted before I started and bought 10kg (22 pounds) of opaque silicone. But seeing that mould having held its shape I might just try going thicker than originally intended.
Maybe I'll try incorporating some very open weave fabric mesh into it?

 

[JPH]

Thankyou for the advice! I wish I had posted before I started and bought 10kg (22 pounds) of opaque silicone. But seeing that mould having held its shape I might just try going thicker than originally intended.
Maybe I'll try incorporating some very open weave fabric mesh into it?

The silicone you bought won't compress, the imortant thing it to have a rigid outer shell.
Create a plane of your subject using clay, then put a wall around it with the rigid outer shell material.
Pour in your silicone to cover the exposed part of your subject.
Then flip the entire structure, clay, subject, rigid outer shell and silicone. Remove the clay.
Generously apply release agent.

Pour second half of silicone.

Consider adding funnels to bottleneck pieces like fins.

Then pour final layer of resin.

 

[JPH]

the bottlenecks are the fins and the tail, areas that narrow and then widen, which can get clogged by curing resin/silicone and result in their tips/ends not getting casting material. Consider using straws or little funnels to those areas.

when I cast the ship I posted, I poured the wings first and separately.