This is a piece I've been working on, at work, I'm thinking noone will be having any direct aplications at home, but some of the techniques may find use.
There are many constraints aside from the obvious. the mold needs to survive thousands to tens of thousands of fill/demold cycles. Everything must be repairable. No part should be able to roll off a flat topped workbench. The silicone develops about 15 psi when injected, so a mold this size needs clamping to resist about 3/4 ton of pressure.
The product is a life sized head skin with attached trachea and esophagus all proportioned correctly, in one piece and airtight, with a constant wall thickness of 1/10 inch. The sunken flesh of the model and choice of parting lines gives substantial interlock of the molded shape. The mandrels (cores in casting a to mold the trachea and esophagus, then lined with modeling clay to represent the skin thickness. The mandrel that forms the mouth interior from the lips to a few inches past the divergence of the esophagus/trachea was screwed in place and clothed in a thickness of clay. (That mandrel was scanned from a previous piece, enlarged digitally, and printed in stainless filled bronze.) The extensions of the esophagus and trachea were carved in steel to match prior examples, clothed in clay, and propped in place. Clay was then added to fill the front of the throat, the bulge of the cheeks, and the dome of the forhead.
The biggest of the mandrels, forming the back of the neck and the bulk of the head was formed in three parts-- a front, a back and a joining slice. The front and back were made in heavily filled epoxy, about 2/3 of the volume crushed walnut shell, wood flour, iron oxide and milled cotton. Separate pours for the front and back cut the heat buildup. After the epoxy kicked, but was still a bit green, the two shells were bolted together and the middle slice was poured with a more fluid mix. A liberal application of a trustworthy release is essential to avert disaster. I used 1:2 lanolin:vaseline.
The rest of the pieces were done in single pours, but with similar technique: clay to line the shell, made parts in their places, clay for parts not yet made, pour into the void. Two of the pieces had sections only 1/16 inch thick, so they got reinforced with copper screening.
In use, the back-of-head and sub-chin mandrels are gripped by the base of the shell, and hold all the other epoxy mandrels. The forhead slab, the point of the chin, and the cheek prominences are held in place with magnets. The piece between the trach and esophagus is captive between the sub-chin and the back. The mouth mandrel supports the esophagus and trach mandrels with sockets, esophagus and trach tails seat into the epoxy mandrel set.
Setting up to demold the part quickly, safely, and without damage is a big part of the process. Most of the damage to molds we see is the result of the demolding process. After unbolting the shell and removing the mouth mandrel screw, a screw at the nape of the neck is backed out and compressed air is run in. After the back is loosened, it is pried off with spreaders. Compressed air is fed to the mouth screw hole to loosen the front. The mold is then bolted to a platform and the shell is pulled off with a screw rig. As the shell comes off, the bulk of the mandrels come first, leaving the magnetically attached mandrels behind. Once the big mandrels are clear, the smaller ones can shift out of interlock and come out with the skin.
With the shell stripped off, the skin is everted over the mandrels, which are subsequently pried apart with spreaders. The metal mandrels are then slipped out and the head flipped right-side out.
Metal form for the mouth interiormouthform.jpg
esophagus and trachea mandrels. The branching to the lungs necessitates the trach mandrel break down for demolding.trachandesophmandrels.jpg
Metal mandrels in position with some of the epoxy parts left out for seeingmetalmandrels.jpg
base of the shot mold showing filler tube and end of esophmoldbase.jpg
back shell pulledbackoff.jpg
spreaders loosening the face sidespreaders.jpg
bolted to table, pulling the face sidepuller.jpg
skin rolled up mandrel set to allow mandrel removal, interior of casting still inside mandrelsskinflipped.jpg
skin still inside out, interior parts freed of epoxy mandrelsheadeverted.jpg
ends of the interior parts with the metal mandrels pulledopenings.jpg