Filling Hollow Resin Casts: Help

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ILL GREEN

New Member
Hi everybody :)

I am currently working on a project molding and casting 10 inch figures for a promo. Figures has to be light for shipping so I'm rotocasting them. My dilema is that the figures feel prone to break. One shattered like glass when I dropped it accidently. Perhaps let me explain my process.

I lay up my molds first with PU resin to sink into the detail, then as it turns to paste, I pour another ounce or two of PU and seal up the mold to be rotocasted. The results are excellent and bubble-free but feel fragile.

I would like some knowledge from you guys as how you fill hollow casts. I thought about foam but worry that it'll expand out and crack my sculpts if overdone. I hear plaster is another method but that will make shipping cost higher due to weight. Should I drill a hole in my cast and pour more resin?

Thank you in advance.

I am also a rookie at molding and casting with a year of experience, so excuse my newbie behavior :)
 

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Alaneye

Well-Known Member
You could fill them with something, but they would still shatter if dropped. I use a similar method to you, but with the rotocast layer I add filler powder which saves a bit on resin and thickens it. Why don't you try adding more resin for the rotocasting stage to produce a thicker casting?
 

replicaprops

Official Licensee
RPF PREMIUM MEMBER
I would say use a expanding foam on a test casting and see where it cracks and then just re enforce that inner section.
 

ILL GREEN

New Member
Thank you Alaneye and Replicaprops!!

I've thought about adding a thicker batch with marble dust and chopped fiberglass in the rotocast stage. I'll give both methods a try and come back.

Thanks again!! :D
 

mslz22

Sr Member
RPF PREMIUM MEMBER
What resin are you using to rotocast with? That may make a difference. Also i have found that some 3 minute resins do not laminate well, meaning that the subsequent layers may not have a strong bond with the initial layer.

Expanding foam is a good option. On rotocast stuff I will add it for strength. few tricks, leave the part in the mold if you can, that will help with eliminating any distortion that the heat from the foam may cause. Slowly figure out how much foam you need, start with an estimate amount by figuring out the estimate volume you need to fill, then dial it back a bit. Once you dial in an amount write it down and stick to it. Usually it's in grams so you can be almost exactly correct, ie 25 grams part a 25 grams part b.

Lastly you may want to consider shushing the foam around the inside edges for backing rather than feeling like you need to fill the whole thing, you can have a hollow "center" it will buy you a little wiggle room as a hedge against over expansion.
 

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ILL GREEN

New Member
What resin are you using to rotocast with? That may make a difference. Also i have found that some 3 minute resins do not laminate well, meaning that the subsequent layers may not have a strong bond with the initial layer.

Expanding foam is a good option. On rotocast stuff I will add it for strength. few tricks, leave the part in the mold if you can, that will help with eliminating any distortion that the heat from the foam may cause. Slowly figure out how much foam you need, start with an estimate amount by figuring out the estimate volume you need to fill, then dial it back a bit. Once you dial in an amount write it down and stick to it. Usually it's in grams so you can be almost exactly correct, ie 25 grams part a 25 grams part b.

Lastly you may want to consider shushing the foam around the inside edges for backing rather than feeling like you need to fill the whole thing, you can have a hollow "center" it will buy you a little wiggle room as a hedge against over expansion.
I am using Model Pro Slo polyurethane resin from Specialty Resins. It has a 6 minute cure time with an hour to demold. I use it for both lay-up and rotocast. Good stuff. I should've gotten the 65D Tough Cast for the lay up but oh well.

Foam I am looking into but my molds are closed-molds. I was thinking of drilling a hole into the cast and pour it in. However, some casts are thin and I fear adding foam or resin will deform my sculpt due to the exothermic heat they give off. I tried slushing it with small amounts of resin and that has been keeping the heat controlled and builds reinforcement but its takes a while with the 6 min resin and I have 20 sculpts to reinforce.

I like your idea of a hollow center, that sounds on the money. Thank you mslzz22! I'll try that.
 

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