Undercuts and hollow pieces.

With one piece like that, people usually don't use a core mould but they add resin and spin in, coating the inside of the mould and gradually building it up.

 

I have no clue how they do a one piece torso other than what Ecchan said, which is technically called a "slush cast". I do multi-piece torsos so that they can bend and stand better, and because it's easier to cast. :/

For a slush cast, the easiest thing to do is to get some sort of centrifuge, but that's expensive. The poor man's way is to fill your mold and then start turning it by hand. Plug the pour hole with your finger and just start turning! you'll make a _mess_ but it will work. You might have to do several layers to make sure it comes out thick enough.

 

actually its called 'rotocasting'... there are some places you can pay money to use this service... i'm actually having the same problem (the 1 peice torso) so what i am going to do is use an old turntable to spin the mold for me.. i'll tell you how it works out once i've finished it.

 

I've been thinking a lot about crank ice-cream makers... If you turned one on it's side you could probably work out a pretty good roticery... I'm going to watch the value village, try to find a five-gallon and try to modefy it. If I get something good, I'll put up a tutorial.

 

Ice cream maker will only spin your mold in a circular motion. You want to move the mold in all directions or it won't work.

WHen I do hollow cast pieces, I do not put in pour holes. I pour the resin in one half of the mold, place the other half on and wrap the whole thing with heavy rubber bands. Then pick the mold up and start rotating. It takes a while for the resin to set. I usually have a small mold that I pour some resin into at the same time so that I can see when it has set. Once the sample piece is set, I usually spin the mold for a bit longer, just to make sure. I then let the mold sit for a little while before opening it. The freshly cured resin is very soft and you can easily crush it.

 

hmm...well if its that easy...lol do you think i'td be bad to use resin that sets up fast ( the kind that cures in like 10 minutes) to keep the rotating down to a minimum (so my arms wont get tired! :P )?

 

Damn, so to mechanize it I'd need one of those spinny things with the one seat in the middle like they have at fairs... hm...
Do you rotate very slowly? How do you avoid bubble when you're doing this?
Also, what brand of resin do you use? What's the viscosity like?

 

Maybe you could find a mechanized rock tumbler, like in one of the little kiddie science kits?

 

Same problem. Rock tumblers just rotate, also, that would probably give you a lot more room for bubbles. And the cheep rock tumblers are tiny.

 

The gyroscopic ride at the fair is a perfect example of the motion you need to replicate!

Years ago I ran a small company that made resin model kits. Most of the parts were solid but the couple hollow pieces that I made I did like I outlined above. The resin I used at the time was Vagabond Odorless White. It sets up in about 5 minutes or less, which is why hand rotating a mold is possible. If your resin takes longer to set, you are in for one heck of a workout! Silicone isn't light!

As far as silicone goes, the best available for resin casting is called GI-1000. I have cast over 50 pieces from one mold with little loss of detail.

 

alright!! i'm going to do the rotocasting by hand! the Smoothcast (?) resin that i bought sets up in 10 minutes (more like 5 though) and would be perfect!! Wish me luck, as i'll be molding and casting my doll this weekend!!

 

That's what slave-labor --I mean-- devoted friends are for.

 

Hey! i finally tried Nepenthes's method of hand-rotocasting on a torso!! it WORKS!! perfectly...! i think i'm going to hand-rotocast the rest of the arms and legs to create hollow peices... its just way too easy to pass up!

a picture of the original next to the copy:



THANK YOU NEPENTHES!!

 

I thing tehy glue together back and from pices of torso

 

Glad to hear that it worked!

 

I can't even tell which one is the original and which is the casting... Man, I would have thought there would be terrible potential for bubbles with this method...

 

The constant movement dislodges any air bubbles. Your biggest worry is not getting the molds seated correctly, but with good keying, it isn't too hard.

 

Wow, that's awesome. *takes notes*

 

is there an easier way of ensuring a constant thickness on all parts of the cast? I've done 3 torsos already, but i'm having a problem where sometimes the lowerhalf will be thicker than the upper half, or vice versa... and i'm trying to rotate everything evenly... is there anything i'm missing? maybe i should pour less resin into the mold?

 

That is always going to be an issue unless you do an interior mold.

 

I've got keys about every three to five centemeters. What I'm trying to figure out now is if there's any good way to do a head with this method and still have it able to match up neatly to it's scull cap, or have the eyes come out nicely. Do you do heads with rotary or a plug? Also, what's the best way to open pieces larger than a drill bit? I've got the bottom of the torso piece and i have molded it so that the hip piece can fit in without opening it up, but making it a wider opening would be a heck of a lot better for stringing... I was thinking a router (with a reeeeealy fine bit so i don't horribly chew up the resin), do you know of anything better?

 

Yeah, I didn't really think so. And the head is about the easiest plug to do anyway so I probably wouldn't bother if this was a possibility.

Now I have a question on carving through the little bits that have covered in resin though, like the long, thin cannals for shoulders and thights and such. What kind of tool does one use there? Something like a teeny tiny router? Is diamond the way to go so as not to shatter tiny pieces?

 

Oh, I already have that, it's the thin piece I was thinking about (and actually, I'm thinking i may clip out the inner core and modify my mold a bit to do the rotary method instead of how I'd planned). I asked my mom today and she said that's dremmal work, so dremmal it is. The Brother in law cautions that I need to put it on a slower setting, so as not to create enough friction heat to melt the resin because then it turns a dremmal bit into a smooth cylindar pretty fast... And my mother already has a dremmal tool that I can borrow! Hazzah! Something I don't have to spend more money on! I've had pretty good money mojo today...

 

If you mean like the seams and the vents... when you pop your piece out the resin will still be warm and slightly soft, it's easiest to take an exacto and cut off any extra parts then. It's also the best time to take out the core molds while the resin is still warm and soft but not /squishy/. Else you run the risk of tearing your core mold no matter how smooth it is.

oh! And your dremel can be used as a drill. there's a line of drill bits for it but regular small drill bits will work too. But it won't take all sizes, just up to I /think/ 8mm? Not sure about that off the top of my head. you'll need a high rpm for that tho. also, for doing long portions there's a type of drill bit called an "airplane" drill bit? I think? It's extremely long and good for if you need to drill a part that's deep in an arm or something. :3

 

Okay, so I've tried to do the rotary method a couple times on fairly thin pieces and it keeps ending up with almost all the resin lumped on one side and a paper-thin layer on the other. I figure it must be a problem with my speed of rotation. Can anybody tell me how fast I should be rotating the mold?
It sounds like the brother-in-law might make me an electric rotator/tipper once I figure out the specs for it, if it is successful, I will post the plans up here.

 

i found that i could only do the rotary method on the torso peice... and not on the arms and legs... i'm not really sure why, but i think it has something to do with the airbubbles moving more slowly in tube-like casts...whereas with the torso it was more spherical, and less chance of the airbubbles being stuck in an end of a tube........ if that makes any sense...

 

It may be worth trying to rotate the arm and leg molds along the long axis as opposed to every direction.

 

I was thinking that was probably the case. I think I've figured out a way to modify my existing molds for a regular pour casting in a way that makes sense to me though, so I may not have to redo them. My main issue with pour casting them was that the cores in arms and legs are so skinny, they flop over and it's troublesome for keeping them straight. But I think I've got that figured out now, so I'm going to give it another go.
As for straight rotation rather than tipping, I did try that. Didn't work. Thanks though.

 

Oh I had ment to post! I came up with a method of making a piece like this hollow, and it doesn't take any special equipment, in fact I used a Styrofoam ball, bubble tea straw, and press and seal.

It worked really well and made a hollow no drilling needed torso piece. I have some more casting to do so I'll try get a photo of the set up for you.

Also managed to get drill free, or almost drill free arms and legs too using the my drinking straw method.

 

Does the plastic the straws are made of not stick to resin? I was thinking of using something straw or pbc for a core, but i thought the resin would stick to it.

 

Nope it doesn't stick. Once i make my masters, I put little bits of drinking sraws in the ends and make the molds, then stick a drinking straw in the gaps for them in the mold and pour the resin in.

The resin sets hard around the straw, but not in it if you get them in right, after casting with some tweezers twist out the straw and you have a nice smooth no drill needed hole!

For the pieces like in the start of this post I do the same thing whilst making the mold, only when casting I make a Styrofoam core, attach bubble tea straws (where the holes need to be bigger like the legs) and wrap the Styrofoam in press and seal (clear not colored)

The straws then go into the gaps in the mold same as with a straight leg piece, only this time they hold the Styrofoam core in place.

Pour the resin as before, and twist out the straws when set. Then go outside, drop a little nail polish remover with acetone down a hole, and that will get rid of all the Styrofoam, leaving you with a hollow torso!

If you have a straw that doesn't have a place to connect at either the top or bottom of a mold, take some waste set silicone form the mold making and block the open straw end with that to stop any resin getting in.

I swear it works a treat!

 

I ended up using small drinking straws when I was making the molds to make hollow tubes and then putting scewers up to keep them ridged while I cast the resin. It actually worked pretty well, and next time i think i'll be able to get a much cleaner mold now that I've figured this out... Although I do think I will run up to the Asian market and get some bubble-tea straws as well, having the inside of the limbs that smooth would be nice. Thank you! You have provided me with so many good ideas through this process (the legos is still my favorite... sheer genious!)

 

Always glad to help! I just wish the legos had been my idea, the man at the hobby shop told me about them!!

 

this is all great info about casting, thanks! X33 I like the straw idea!

and dremel drills kick ass. I love mine *pets*