Bubbles in my finger molds.

[TrueDave]

I cant seem to get this. I've squeezed, bumped etc. I get castings missing fingers and toes. I can build them up with Squadron Putty but it's too fragile.

"Pressure Pot" ? I have never used one. I dont think I can afford one either.

Anything OTHER than a pressure pot?

Thanks Everybody

[TheDoLittle]

I'm assuming resin/plastic casting...

If I'm not spin casting, I made a little vibration tale with a large piece of MDF, sponges, and a discount back massager. Advanced Casting Instructions

To be fair, I've never cast anything in resin larger than a /172 scale spitfire land gear and engine, so I don't know how much a vibration table would help.

[Tothiro]

Regarding fragility - the cast fingers will be fairly fragile too. It's an unfortunate problem with resin casts, they don't have the give that the modern repros do and they aren't as solid a material as vintage. They work ok if you can get them bubble free though.

Your three options really are pressure pot, vacuum chamber (to de-gas the mix before pouring - but again money... unlike pressure pots though they're safe enough to tackle as a build-it project and would run about $150-$200) Third would be resin strengthening additives, but you have to get the bubbles problem solved.

For hands and things with thin cast sections I'd say start with a bottom-fed mold. A large number of the bubbles that get trapped are caught up in the resin as it's pouring in, so a bottom feed mold would alleviate some of the issue.
The set up is fairly simple - instead of attaching your part on a stick to point upward when you pour the mold, attach it to a curved, J shaped stick to point downward... like a drooping flower. This way when you flip it and are pouring the resin, it's flowing down underneath the piece and then rising upward from the weight of incoming resin behind it. A lot more air gets shoved up and out of the finished piece this way.

[megowgsh]

Post a picture of what you are casting along with how it "sits" in the mold. Then we can give you advice on how to best cast it.

[TrueDave]

I was just thinking of a bottom fed mold this morning. Wouldnt you have to have an air vent for the bubbles to escape?

I'm just trying to cast some old Star Wars 3 3/4 arms. the thumbs and knuckles are teh problem. Same thing with my DC Direat Gorilla Grodd hands and feet.

I'll try to get some pictures up thank you all.

I know most resin is too brittle for putting accessories in the hands but I have a couple of containers of a less rigid type of resin I still havent cast with yet.

[Tothiro]

I was just thinking of a bottom fed mold this morning. Wouldnt you have to have an air vent for the bubbles to escape?

Yes, exactly - and you can either cut those air vents out of the mold halves after curing, or you can put them in before you pour the silicone as additional support for the piece. I find I do both, depending on the item. Thumbs and fingers with a curvature are more complicated, because they'll catch air... you just want to make sure the fingertips will be positioned pointing straight up in the final mold when it's inverted for casting.

This is a simple cast because there's not a lot of air pockets, but its bottom fed, with the resin coming up through the center of the wrist ball joint. I mounted the piece on a bent wire and built out a slope with kleen klay to give it lots of room for resin to pour in, because just like a siphon, the more resin behind it - the more resin is under pressure to push up and back out the fingertip vents...

I know most resin is too brittle for putting accessories in the hands but I have a couple of containers of a less rigid type of resin I still havent cast with yet.

It just can't really hold tension or flex, so if you've got something gripping it's an issue, unless you just have items slide into the hand from the top or bottom. Anything that has to give and then "pop" back around an object is going to break in traditional resin.

You can get some very complicated casts using bottom fed molds without a pressure casting though... This gun has tons of bottlenecks in the mold and is very thin in some barrel sections and such.
..

[David Lee]

Pressure tank can't be beat...
$80 at Harbor Freight... also try Craigs List for a used one... Oh and you need an air compressor too

-Dave

[ctc]

Hmmmm....

I vent the hell outta them. Looks weird when there's more flash than part, but it works.

Don C.

[TrueDave]

Post a picture of what you are casting along with how it "sits" in the mold. Then we can give you advice on how to best cast it.

[TrueDave]

You can see my issues here.

The venting you guys are talking about reminds me of the lost wax method. Can I see a mold with one of these vented ETC. examples?

The top two pics above are Star Wars arms , theres my concern for breakage if I just putty them. An accessory would pop it.

I HAVE been playing with a funky resin from Industrial Polymers, TrueCast 65. I got in the hopes of making my own boil and pop type Swivel Biceps.It might be flexible enough for a gun etc.

Pressure Pot. How strong an Air compressor are we talking about? Last one I ever had in my home studio was pretty small, just for an airbrush.

Third Pic is a custom Mego pair of claw arms I really want done. I'm thinking about webbing the fingers and then cutting it out after casting. Picture what I'm thinking?

do you put items in the Pressure pot when youre molding them? Will my molds work in someone elses Pressure pot?

My wife asked if you could clarify this part Tothiro
" you just want to make sure the fingertips will be positioned pointing straight up in the final mold when it's inverted for casting."

I'm curious how you close the mold with the bottom fed channel. Can I see a cast piece before you clean it up?


Thanks for all the help everyone!

[Tothiro]

do you put items in the Pressure pot when youre molding them? Will my molds work in someone elses Pressure pot?

Yes, but pressure pot intended molds will probably have to be cured under pressure as well, otherwise any latent air bubbles in them will deform the mold when you fire the pot up for casting.

My wife asked if you could clarify this part Tothiro
" you just want to make sure the fingertips will be positioned pointing straight up in the final mold when it's inverted for casting."

I'm curious how you close the mold with the bottom fed channel. Can I see a cast piece before you clean it up?

Sure...


A bit out of focus, but you get the idea. Essentially you want to make any small extremity that would become a trap for air vented at the top most tip...
If you're doing something with an extreme C cup shape like those SW arms though, you'll probably want to run a vent from the tip of the thumb up into each of the fingertips, and then vent for air out of the finger knuckles on top... (presuming the shoulder sits at the bottom of the mold)

[TrueDave]

YES! Thats how bronze class did it. Can I see what the mold looked like?
The bronze one was destroyed. Not reusable.

I understand the mold would deform if not curewd under pressure. * lightbulb*
Last time I messed with air compressors was with airbrush. Noisy.

Was the Spock hand pressurized?
Do you drill out the wrist ?
On my feets I slide a brass rod through the mold before I pour then pull it out after . It leaves a clean hole.