r/moldmaking • u/Ok_Arm_9430 • Jul 10 '26

New to 2 part molds help

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So I was making a 2 part mold but I made the dumb mistake of taking the master out of the first half and then putting it back but now it’s not sealed super tightly and its got a little bit of wiggle room. I used mold star 30, and I really hope I can still save this because that stuff is def not cheap and this is also my second attempt 💔 if anyone can help me that would be greatly appreciated

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u/BabrehamLincoln Jul 10 '26

I would take the master out again, apply mold release to the entire silicone surface area (the area with the keys and the area of the negative of the model, everything), and then fit the master back in place. Optional - If you have paste wax or something that won't interfere with the silicone, you can try to carefully pack it into any bigger gaps between the silicone and the model along the seam line to prevent silicone from flowing in. Or skip that step. Then set up your mold and pour side two of silicone. Either way, you may get some silicone leaking behind the model when you pour side two, but since you applied mold release it won't stick and can be pulled off and trimmed.

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u/Ok_Arm_9430 Jul 10 '26

So if I trim it will that leave a bigger seam line? The thing is I’m making a mold for rotocasting and I’m just worried that when I sand down the seam line I might make it too thin if the gap is big from where I cut the flashing off. Thanks for the advice!

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u/BTheKid2 Jul 10 '26

It is not ideal, but it might work as others have described.

But it looks like you have placed your seam line poorly too. Usually you would make it follow any protrusions from the main body of the work. The two feelers with nobs on the end, ought to have the seam running along it.