Avoid Pooling when Slush Casting


Active Member
I've recently done a couple of cast of a Borderlands 3 Psycho mask, using slush casting. I'm using Smooth Cast 300 for the resin. Problem is when the resin really starts to set, it starts to pool and form in one spot, as I'm sure has happened to most people.

You can see the ugly and uneven surface on the back, luckily this side isn't on display! Stupidly I didn't make a lip before making the mold (I thought I did but I got my geometry wrong) so going up to the edges is a little tricky, but that's a problem I know how to prevent for next time.

What tips do you guys have to avoid this pooling when slush casting? Cheers.
Are you doing all that as one big pour? If so you should do several smaller pours. The first will just skim coat the surface. Do the next pour as soon as the resin flashes, not too much later. Each subsequent pour adds a thin layer on top. Keep rolling the mold around as you go. This will keep a large amount from creating a glob.
I did four layers of about 70-80 grams each (I know it's kinda hard to tell if that's a lot). Should I be doing more frequent and smaller layers? Also, by "flashes" I assume you mean when, in the case of Smooth Cast 300, it turns white?
The more mass, the faster it will set. You want to keep the material from bunching up. The trick, is to rotate the mold at the same speed the material flows. Too fast, or too slow and the material stays as a relative single mass. If you stop before it sets, it will also pool. Try mixing a batch in a cup and observing it's cure characteristics. How fast it moves, relative to it's set up time. This will help you judge how fast to rotate your mold to keep the resin from staying in one lump. If it's an open mold, or has a large pour hole, as soon as the material starts to thicken, turn the mold upside down and let the excess material drain out. Then, as "Valor" suggested, add additional coats as needed.
Smooth Cast 300 is a fast setting resin, so it is more prone to that. When it "goes", it goes fast!
A slower setting resin that sets more progressively will not create globs as much.
Slower setting resin that sets more progressively is not something I have ever found in casting resins. All polyurethane resins are exothermic in nature, so slow or fast, the places you have more volume will set faster, regardless of the pot life of the material. Unless someone is now (past 5 years) is producing something with a significantly different cure curve, Pot life just means you rotate your mold for 3 to 5 minutes, or 30 to 50 minutes, but you still have to rotate the mold at the proper speed.
Years ago we used BJB's 1630 resin which had a slower, more progressive gelling action. It gradually thickened, which allowed us to brush it up the sides of molds to create hollow helmet castings. It wasn't immune from a globs forming if a puddle was allowed to form, but the way it gradually thickened made it nicer to work with than faster setting resins.

Imgill is right about puddles of resin creating a place for a glob to form.
A large volume (like a puddle) allows the endothermic reaction to localize the heat. This hot spot accelerates the curing of the resin in that spot.
I've only got a little experience in slush casting resins, but so far the results have been pretty good. Looking at yours, I think you may be using too much per coat. By the time the resin starts to set, there should be very little that's still slushing around in the mold; most should have already coated the inside of the mask...so it sort of looks like an inverted candy apple. If you can see into your mold while you're doing it, the last bit before it really starts to set should feel like a few drops just barely making their way around. It looks to me like there was still a ton of resin left when yours started to set and the rapidly setting resin has sort of folded over itself like magma. I think you'd be better off underestimating the volume needed per pour than overestimating. If you feel like your resin is starting to set and you've still got way too much, you could always pour the excess off into a bucket. It's a bit wasteful, but if you're just going to have a ruined cast anyway, it's probably the better option
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