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!)
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!)