For puppets and prosthetic appliances, carved foam rubber tends to look like, well, carved foam rubber. A properly sculpted and refined shape can be rendered in soft, flexible urethane foam in a relatively simple process utilizing basic mold making methods and materials. The softer and more flexible and compressible the material, the better. The artificial skins on Hollywood animatronics and puppets are often much softer than they appear so as to make them respond realistically to their limited range of operation. Dead weight can be made to move like live muscle and subtly respond to the operators/puppeteers.

This Instructable covers the construction of the snout for a donkey mask.
With an effective donkey mask you can:
-Play a practically-themed joke on a farmer
-Scare your dog
-Promote the Democratic Party
-Rob a bank
-Make an ass of yourself
-Get a discount on your entrance to a furry convention

This project was created at TechShop.

Step 1: You Will Need:

-Scrap wood and basic woodworking knowledge
-Two-part silicone rubber
-Two part soft urethane expanding foam
-Release agent
-Clay
-A fine-toothed sawblade
-Proper mixing containers and tools

Step 2: Sculpture

Referencing hundreds of photographs and one real donkey, I arrived at a three-dimensional shape which suited my purposes. I sculpted a three-inch clay maquette, focusing on getting one side perfect. Using a the 3D laser scanner at TechShop and Autodesk 3dsMax, I produced a mirror of that side and achieved a symmetrical donkey shape.

Step 3: Into the Real World

Using the CNC ShopBot at TechShop, I milled both halves in MDF (medium density fiberboard) using a 1/4" ballnose bit (I used a store-bought 1/2" router bit for the roughcut.) The parts needed only a light sanding and some finish work around the eyes and nostrils. I also added a coat of gesso to seal and smooth the MDF.
With both halves complete the donkey head takes shapes in the world of the physical.

Step 4: Casting the Snout

To create a decent and stable negative mold of the snout, both halves have to be clamped together and suspended over the mold. I made a wooden box, just slightly larger than the snout, to contain the mold material, in this case, Smooth-on OOMOO 25, a two-part silicone rubber compound of medium stiffness. It will be soft enough to allow release of the original part, but stiff enough to hold its shape with the help of the box.

Step 5: Add a Release Agent

Immediately before adding the urethane, apply a thin coat of release agent to the silicone mold. You will not need much, as silicone rubber tends to not stick to anything. Be careful not to allow any of the release to pool at the bottom of the mold, as this will affect the final part. Just add enough to make the mold slippery.

Step 6: Mix the Urethane Foam

The final snout part will be made from Smooth-on FlexFoam-It 3-pound foam. This is the lightest and softest foam of its kind, and when mixed correctly, will expand to 16 times its liquid volume to weight only 3 pounds per cubic foot.
The two parts are to be mixed in a 2-to-1 ratio. A fast a thorough mixing is necessary for optimal results. I am using an electric mixer on a cordless drill. The foam has to be poured after 30 seconds then left to its own devices.

Step 7: Watch the Urethane Expand

Behold:

Step 8: Release the Part

The manufacturer recommends a 30 minute cure time, I waited 40 minutes to be on the safe side, as a an early release can cause a soft mold to collapse.

Step 9: Cutting and Trimming

Foam this soft has to be cut with a very fine and very sharp toothed blade, like from a new hacksaw. Do not apply much pressure, gently pull the blade back and forth without compressing the foam. Using a straight edge or scissors will cause the foam to give and deflect and this will not produce a predictable cut.
Here I am cutting open the mouth and hollowing-out much of the shape to create an even more responsive part.
The superstructure was vacuum formed in PETG plastic over the MDF forms.
Urethane foam can be painted with airbrush paint, either using an airbrush or a standard paintbrush. Hair can be punched and glued into it as well.