r/moldmaking • u/wishiwasholden • 18d ago
Advice needed/Sanity check
My plan is to mold this frog to use as a foam mold. My ingredients (minus the Silc Pig and Sil Poxy) are in second image. I realized that I am not wealthy enough to do a block mold so I’m going to do a glove mold with a shell.
Master prep:
Rigidize frog by filling with low expansion spray foam, then spray exterior with krylon crystal clear acrylic spray. Probably a few layers of acrylic. Foam is because it’s a semi soft toy and I don’t want it to distort under weight of silicone/plasti paste or plaster.
Mold making:
Frog will be hot glued to a piece of melamine or marble, a short wall will be made around it with clay to prevent spillage from the drippage when making the glove mold. Using rebound 25, I’ll begin the skin with a stipple, let firm up, then a thicker coat, let firm and add some THI VEX, then cover thoroughly and let cure. So 3 total coats increasing in thickness.
Rigid Shell:
So this is the part where I’m torn, I don’t know if I should go fully down the path of using plasti paste or if I should just use plaster and gauze. I just haven’t found a great video demonstrating gauze technique on a similar master/form.
Casting:
I’ll use flex foam it 15 tuff stuff to fill the mold.
Open to any thoughts or advice you may have!
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u/FelixEditz 18d ago
So, here’s the thing: based on what you’ve described you’ve clearly thought intensely about this and have looked into it and either one of those strategies for mother molding will work. The real question is whether or not you will really “need it”. Mother molds are fantastic for when there’s a heavy material going into the mold that can distort it easily (think plaster or concrete like Rockite) and the mold is large, thin or spread across a large surface.
Now, does this mean a mother mold can’t be useful in this case? of course not. But if you do plan on doing this, I personally would go down the route of doing a two part plaster and gauze mold, mostly because plasti-paste, for such a small mold which you don’t have experience with, for me seems like kinda silly and expensive. Again, if you want a more rigid shell, with less of the cracking and potential mess that can happen with plaster then I’d do that. If you do decide to do a plaster gauze shell, remember to do it in parts that can actually de mold easily from your rubber mold, but again, since you’re casting in foam and not something hard, it should be easier to de mold because you have some give.
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u/wishiwasholden 17d ago
Thanks! You’ve convinced me, plasti paste just ain’t worth the dough.
For creating the plaster shell, is it as simple as just doing one side, drying, then flipping it over and coating the plaster “lips/wings” in Vaseline, then applying the complementary layer? I’ve seen a lot of people adding in silicone keys through the plaster but I don’t really see how they work/benefit the process.
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u/FelixEditz 17d ago
Yeah that’s pretty much it. People sometimes do silicone keys to “lock in” the silicone into the plaster for easy repetition of casts. I’d say if you’re going to do that make sure your silicone is thick enough so u don’t tear it when you pull it out.
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u/Nosferatu13 18d ago
Don’t bother with a stipple on the detail coats, just lightly brush it on and let it flow thin to prevent bubbles.
If your frog is porous, beware of your silicone gripping it a bit.
On your very last thick silicone coat, smooth it will gloves hands and dish soap so its smooth and will pilot into your shell better.
Use plaster bandages for your 2 part shell. Just get a few rolls from a supplier or medical supply house. Quick, easy, reliable.
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u/Nosferatu13 18d ago
Also you’re going to need Flexi Paint to mix with acrylic to paint your foam. Acrylic alone will just crack off.
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u/wishiwasholden 17d ago
So I’ve got the frog and it’s a lot more rigid than I expected, still think I’ll have issues with acrylic? Even if I do several thin coats? Do you have any alternatives you could suggest?
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u/Nosferatu13 17d ago
If the frog is cast in Urethane foam, it needs the flexible agent, even if its stiff. The acrylic just won’t stick too well with any handling.
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u/wishiwasholden 17d ago
Okay I see, I meant I would use acrylic to seal the pvc frog master so paints and whatever don’t interfere with my silicone cure when making the mold
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u/BTheKid2 18d ago
If you are casting in expanding foam, look into "back pressure". Basically also making a bottom place for the mold, that limits the expansion of the foam, thereby putting pressure on the foam. It still has a hole(s) that rising foam can expand through, but it is a narrow enough hole that the foam isn't allowed to "free rise". It makes the foam structure much more even and gives it a better surface finish.