Thread: Lots of skulls from liquid plastic and foam

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I wanted to get a lot of skulls this year (like minor The Killing Fields quantities) but I didn't like the look of the commercial plastic 'Halloween' versions and at $15 a pop the 4th quality anatomical ones would be too expensive. (Not to mention heavy-- they're close to 3lbs each!)

Casting my own in liquid urethane plastic and foam seemed like a good solution. This is what I did:

Start with one of the $15 "4th quality" anatomical skulls available online:



I didn't want to have to make a *really* complex multi-part mold, so the shape needed to be a closed surface-- essentially there could be no "loops" in the skull or else removing the silicone molding compound would be impossible. I started off by using Bondo to seal up parts of the skull and build up the volmer to make it more visible. (When making castings it's easier to have something there and carve it away on the 'copies' if need be, than to try to add it.)



The side of the skull presented a problem since you really expect to see that open "hoop" from the zygomatic bone... But it would be impossible to remove a part from the mold if it was cast that way. To work around that I put a piece of Saran wrap on some clay and gently blocked the base of the 'hole':



Here you can see it from the other side:



I used a tube of 60-second epoxy to squirt in a thin layer on top of the cellophane and once it set, removed the clay backing, leaving a thin skin of epoxy as a barrier.



With the prep work done, I started off with a thin ("detail") coat of silicone mold making compound. (I used the Silicone RTV system from Tap Plastics-- about $24 for a 1lb tub, but Smooth-On stuff or whatever else should work. I like Silicone because although it's more expensive than latex or urethane rubber you don't have to worry about the plastic sticking inside it.) The first coat just gets dabbed on with a brush to get in the little nooks and crannies for fine detail:



Additional layers of silicone are built up to give the mold strength (I added a little pigment to make it easier to see what I've coated with each pass):



Finally, after a few thin coats a little thickener is added to the next layers to make the silicone a more paste-like consistency and the mold is built up for more durability:



Where the mold will be cut, a thick ridge is built up to reduce tearing. The plastic will just pour in the hole in the bottom.



With the mold done, I used some Smooth-On "Plasti Paste" to build a support shell. This keeps the silicone mold from distorting during casting. Plasti Paste is about ~$30 for a trial size and I used maybe half of it for this project. I applied the paste in four sections (taking care to apply some mold release where they need to separate).



(splitting here due to limit on number of inline pictures!)

(and picking up with 'part 2')


When the Plasti Paste cures it's a lighter color and it sandable, drillable, etc. It's pretty tough stuff.



I added some reinforcement 'tabs' and drilled some holes to put carriage bolts through to help keep the shell in place. Since I'm going to fill the mold with an expanding foam that can actually push the mold apart from inside if you don't secure it. If you were only going to cast in plastic and leave the parts hollow you could just use rubber bands.



Once the support shell is done and removed I made a couple cuts in the silicone-- just enough to remove the original skull. I the original can come out, so can the cast copies. Here you see the silicone mold turned essentially inside out. The silicone captures *all* the features of the original-- right down to fingerprints if you left any!



My casting technique used the basic urethane plastic "Quik Cast" from Tap Plastics. Just so happens it's a 'bone' color. ;-) It's about $26 for the "pint" sized version, but the per oz. price comes down in larger volumes. The plastic mixes 1:1 by volume and I finally arrived at about 70ml of each part for the skull. The plastic is poured in the base and then slushed around to evenly coat all the interior of the mold. Once the plastic starts to set (you can see it start to turn from a cola color to tan) I place the mold teeth-down, so that the excess liquid pools in the teeth to help make those stronger

After about 10-15 minutes (depending on temperature) the plastic will be cured. I then used some of the Tap X-30 expanding foam to fill the interior to make the casting stronger. About 50ml of each part of the foam (~$28 for the quart size) expands to fill the entire skull. Again, I slush the liquid around inside the skull to get it up around the thin bones in the eye sockets for added strength.

The expanding foam takes about ~10 minutes to set up where you can demold it from the shell. I like to remove the casting at this time when it's still *slightly* pliable since it's easier to pull out some of the larger features (like the eye sockets and teeth).



If done right, you get a perfect copy of the original. Since the silicone doesn't need a mold release agent applied, the resulting castings are easier to paint too. Once nice thing about doing 'bones' in this way is that if you *do* have an "oops" and break something off when demolding (or if the plastic didn't go everywhere you wanted it to) it just looks like a normal damaged bone. :-)



I yielded about ~9 castings from my smaller quantity liquid resin containers which puts the cost at about $2.50/ea. If you buy the larger volume plastics it's more like ~$1.50/ea. The mold will last for years, so although it cost another ~$30-50 depending on how much silicone and plastic you use, you could literally cast 100's if not 1000's of pieces with it.

It's not a *cheap* process to get started with, but it produces very nice quality output. Depending on temperature you can do about ~3-4 per hour taking your time. Only about ~5 minutes or so per cycle is actually spent 'baby-sitting' the mold, the rest of it is just waiting for the materials to set.

Let me know if you have any questions or need to see more pictures of anything.

-Clay

[Edit: P.S. The resultant cast parts are about ~6oz where the original cast skulls (without jawbone) are about 45oz! Much easier to deal with when putting a lot together as a sculpture/architectural element. 10-19-2010 -- revised per piece pricing based on current Tap Plastics rates.)

Very cool, you can just drop that whole pile of skulls right in the mail to me Seriously though those came out great, too bad you cant find nice skulls like that in the stores for around $5 a piece. I'd seriously buy some of those for my display. (Hint...Hint)

You make it look soooooooooooo easyyyyyyyyyy,,,
Seriously though... GREAT JOB, and thanks for sharing!

great looking product...I have zero experiance with mold making, but would love to try to do something like this. is there a cheap, quick and dirty way of making a mold for something "halloween useful" that I can hone my skills on without the $100ish investment on high quality silicone and latex stuff? Can I substitute great stuff type foam in my practice molds vice using the expanding foam that you mixed?

thanks for any advice you can lend me.

dK

Originally Posted by DannyK

great looking product...I have zero experiance with mold making, but would love to try to do something like this. is there a cheap, quick and dirty way of making a mold for something "halloween useful" that I can hone my skills on without the $100ish investment on high quality silicone and latex stuff? Can I substitute great stuff type foam in my practice molds vice using the expanding foam that you mixed?

thanks for any advice you can lend me.

dK

I have a latex mold tutorial over here that can be done fore $20 or less.

"Glove" Mold Making 101

Let me be clear, though, you get what you pay for. The silicone mold would last essentially forever. The latex ones, not so much, 2-3 years ish.

This is awesome, thanks for showing us how. One of these days ill muster up the nerve to give it a try.

Originally Posted by DannyK

is there a cheap, quick and dirty way of making a mold for something "halloween useful" that I can hone my skills on without the $100ish investment on high quality silicone and latex stuff?

You can make molds out of lots of different materials depending what your requirements are for the final product (if you need to perfectly reproduce something vs. just make a neat shape or if you need to cast one vs. one hundred copies).

What I started with way long ago (before I had access to any "better" materials) was just plaster of paris. A simple one to do is just make a little mold box (nothing more that cardboard box lined with plastic, or some old tupper-ware) with a 'U' shape cut in one end. Pour in liquid plaster of paris and coat your hand/wrist/arm with vaseline and submerge it halfway in the plaster (using the 'U' to place your arm in). Hold still and wait until the plaster hardens (the toughest part!).

Brush on more vaseline on the top of the (already set) plaster and pour in another batch of plaster on top. (Helps to have a second person helping here. ;-) You can add some gauze or other fabric to help the plaster be a little harder to break.

Once the plaster is set, you can gently pry the two halves apart and pull your hand out and you have a nice mold. Give it another thin coat of vaseline to make it easier to remove the casting.

At that point I'd actually just use a tube of tan latex caulk (usually a few bucks at a hardware store) and smear a thin layer (1/8"?) inside each side of the mold. You don't want too much or it won't cure, but if done right it'll make a nice flexible "skin" casting of your arm that will pull out of the plaster without breaking it. (The cure time can be pretty long on the caulk.) You can then put a few shots of spray foam inside to firm it up or fill it with some styrofoam cut to look like bone, etc. Another thing to experiment with for rubber/flexible like casting is "Plastidip". (Used for coating tool handles, etc.) It's a little more expensive, but probably still under $10.

While not a substitute for the high priced materials in all cases you can still do some neat stuff on a limited budget. Typically you'll have to wait longer for things to cure and have less 'optimal' properties with the final product, but it's worth playing around with.

-Clay

Could you just fill the mold with the X-30 and have a entirely foam Skull?

Originally Posted by GodSpeed

Could you just fill the mold with the X-30 and have a entirely foam Skull?

Yep, that works. Conversely, you can use a little more liquid plastic and leave it hollow and still be plenty strong. Smooth-On makes a version of one of their liquid plastics specifically for rotational casting that would work well too. (It's specifically designed to gel and sticks to the sides of the mold as it sets.)

For this particular skull (supposedly anatomically correct) it takes about 50ml of each part of the X-30 foam to fill it. You'd get about ~35 shots from a 1/2 Gallon set. So that'd be about $1.25/ea if you have a local Tap Plastics (or similar) to pick up the material at and avoid shipping charges. Pretty cheap!

When casting with the foam only you might want to try to make the mold so it's fully sealable (with just a couple little vents for air to escape)-- that way once you dial in the exact amount of material to use you can cap it off and get a denser, closed cell foam to capture the detail. Since I kept the thin bones on the skull anatomically correct I used the plastic 'skin' for strength-- I didn't want to risk breaking a foam-only version but it might be overkill.

-Clay