Leaking mold

[Kettlebell]

Hi, this is my first time posting here so be gentle!

I am making vacuum formed styrene shells for casting concrete kettlebells.

The shells are 0.03 thick.

I put the two shells into boxes and then bolt them together.

Then i pour concrete into the mold.

Here’s the problem:

It leaks. When the some of the concrete leaks out, it ruins the surface finish.

Here’s what I’ve tried:

• glued rubber (innertube rubber) to the faces

• worked ok, but it was not durable enough

• silicone caulking - would not adhere to the styrene

Any ideas?

[Onegoodknee]

I assume you have support features in your box that touch the backside of your shells to limit deformation.
I would probably consider adding some fiberglass directly to the back side of your shells to stiffen up the edges for better seal compression in the areas between your fasteners.

I have had pretty good luck sealing molds using elcheapo santoprene foam weather stripping. A super curved shape and a heavy fill like yours probably needs to use round seals sitting in a channel built into one of your shells. You might have to spray on a mold release on the seals to get the longest life out of them. Also might consider using silicone based seals if you need stiffer seal.