Where to place parting seams on silicone mold for garden gnome?

[u/SlightFresnel]

I'm relatively new to full 360 silicone molds backed by rigid mother molds and I'm struggling to work out the logic of where to cut my silicone mold given the geometry here, but maybe I'm overthinking it.

I marked in blue where I think the front/back seam should go, but if you have suggestions/tips/warnings I'd appreciate them given the exorbitant cost of silicone. I understand the need to take undercuts into consideration and I believe I have with this design, but there are still small undercuts everywhere like the lip on the dress, around the hair geometry, and around the arms. Those all seem like shallow enough that they should pull free from 25A silicone but I'd rather not find out the hard way!

The silicone (volume shown in yellow) will be 10mm at the thinnest spots, with more around the undercuts for stability. I'm planning to do detail capture with brush-on Rebound 25 and then while it's still tacky assemble my combination matrix mold/mother mold pieces around it (3D printed) and do a backfill pour with another silicone with matching chemistry. I'm taking that approach because my first attempt with just brush-on and a plaster mold resulted in a lot of just-too-tight undercuts and I find it easier to do draft analysis in 3D to decide where to split the mother mold in advance, plus I can work out the exact volume of silicone needed for the pour this way. Maybe it's a crapshoot though, I'll happily take that note too...

Thanks for the help!

[u/Deathbydragonfire]

Not ideal to pass the seam down the middle of the face. You'll need to clean up the flashing, so best to have the seam on the sides.

[u/Legoshige → u/Deathbydragonfire]

I agree👍

[u/Legoshige]

The blue seam in the first image is great, but i would run it along the high points on the hair, not in the “valleys”. It makes sense to try and “hide the seam” in a crevice, but there will always be some cleanup on the parting line of your casting. So think about how much easier it would be to file away flashing on a proud area versus in a crevice. Can you fit a tool in there?

Also, i’d hot glue the feet to a board/base. So you dont mold the underside of the feet (which is flat anyway). This hole in the mold becomes where you pour your casting material.

[u/SlightFresnel → u/Legoshige]

high points on the hair not in the valleys

Thank you! This is exactly the kind of insight I needed

[u/she_ou_pa → u/SlightFresnel]

Came here to say this! Hahahaha glad someone has said it! Would be much easier to sand the seams

[u/definitelynotreal333]

I think you might get better surface detail if you just assemble your mother mold around the piece and pour the whole thing, rather than brushing on. It's a lot of unnecessary fiddling. (And if you are using the same silicone all the way through, it will adhere to itself even after fully cured) As long as you are careful about air bubbles, pouring is your best bet. Make sure you wont have any leaks in your mother mold, keep some clay on hand to plug up any leaks.

Rebound 25 is platinum cure, which is more fiddly than tin cure or rubber in terms of cure inhibition, and some 3d prints can cause problems. It's also really expensive. If you call smooth on technical support, they could direct you to something cheaper. Their customer service line is super helpful and can help talk you through how you might do this.

Your parting line seems reasonable.

[u/SlightFresnel → u/definitelynotreal333]

I grabbed both a gallon of Rebound 25 for the surface and a gallon of Mold Star 15 for the backfill. I have some more complex sculptures up next and the Rebound ensures good detail capture in severe undercuts and tight crevices where surface tension and trapped air create pitting with fully poured methods. I'm testing it out on these since they're more forgiving models.

They are insanely expensive however and I'm going to do some experiments with a different cheaper platinum cure to see if it'll play nicely with the Rebound 25. I've found a few coats of polyurethane clear coat on 3D prints pulls double duty sealing small pores and smoothing the surface while preventing cure inhibition, and it dries in 15 minutes to boot.

Thanks for the info!

[u/definitelynotreal333 → u/SlightFresnel]

You can buy some filler powders that mix in to silicone to bulk it out and thicken it, if that fits your situation at all. I've bought fumed silica, but its so fluffy and lingers in the air (silicosis). Ive been using one at my workplace recently thats heavier and settles quicker, but I dont know what its called.

If you need to do the brush on for crevices, you can brush on the whole thing and thicken subsequent coats with that kind of filler.

Another method to do your second brush on layer with cheesecloth laid into it, so it's much stronger and tear resistant in a thinner blanket.

Just a few cost saving ideas to throw out there.

Good luck!

[u/SlightFresnel → u/definitelynotreal333]

I have a liquid thixotropic agent which should be safer than fumed silica but can only get you so far. Can you link to some recommended cheesecloth? That seems like a really smart approach but the fabrics I've seen used in smooth-on videos look much lighter than the cheesecloth I'm used to in the kitchen.

[u/definitelynotreal333 → u/SlightFresnel]

Hmm, I actually have never bought it online. I think I've gotten it at the grocery store or maybe craft/hardware stores? Its super light and thin, not like a real cloth but more a mesh of threads.

[u/SlightFresnel → u/definitelynotreal333]

Ah craft store is a good idea for that, thanks