This is how I mold most of my sculptures for resin casting purposes.
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- surface protection, such as newspaper or plastic sheeting
- a sculpture made of sulfur-free material - I use Chavant NSP Medium
- aluminum foil
- chip brushes - cheap, disposable brushes
- Rebound 25 by Smooth-On
- mold keys - previously cast cylindrical nubs of Rebound 25
- plaster or gypsum bandages
- permanent ink black marker
- petroleum jelly or equivalent
Step 1: Mold Prep
My fox blank, prepped for molding. A release agent is unnecessary since I am molding in silicone from clay. Note the mold wall, hinge depressions, and cheek cutaway outline. Also note the newspaper to protect against messiness.
My new hinges, prepped for molding. Note the mold wall is simply aluminum molded into a well with a flat bottom and sides about 1/2 inch away from the hinges.
Step 2: The Silicone Mold
Layer 1 - I take equal parts (Part A & Part B) of Rebound 25, mix them together in a third container, and apply the first layer evenly with a medium-size, wide-bristle paintbrush.This layer is the most important layer, as this is where all of your details are being captured, so be thorough in getting in all the nooks and crannies as well as eliminating any and all air bubbles. After an hour or two, it is cured enough for the next layer to go on.
Layer 2 - applied approximately 90 minutes after Layer 1.
Mold keys are an important element of a brush-on silicone mold, as they prevent the flexible silicone from flopping around in the hard jacket mold. I made these the previous day by pouring a bit of Rebound 25 in the dot impressions in the underside of a small plastic table.
Layer 3 - applied approximately 90 minutes after Layer 2. The keys have been placed on strategic areas: brow, cheekbones, muzzle, and side edges. I will be adding a few smaller ones around the outer edge also.
Layer 4 - applied approximately 90 minutes after Layer 3. Note that more keys have been added around the edges. The silicone molding process is now complete. Rebound 25 takes 6 hours to fully cure, so I usually leave it overnight.
If I am also molding hinges, then when I am ready to apply Layer 4 on the mask, I mix up a double batch of silicone. I used a little over half of that to carefully pour over my hinges and then use the rest for Layer 4 on the mask.
Step 3: The Jacket Mold
Using a Sharpie or other permanent black marker, make a line down the approximate center of the face. This will be the dividing line for the halves of your jacket mold.
I am using medical grade Gypsona brand gypsum bandages for the jacket mold. I like to use small-ish sections (2-3 inches), triple folded.
Use two long, thin strips of tri-folded bandage to make a ridge down one side of the center of the mask, then make the rest of that side. I used an entire roll for the first half - probably unnecessary but I want this mold to last. I use thinner sections (bifold) on the first layer, especially around the keys to hold them as snugly as possible. Ensure that you eliminate all air bubbles.
I use petroleum jelly (or equivalent) as a release agent for the mold overlap; otherwise, the plaster will stick to itself. Using a brush, I paint the jelly over about two inches of the center divide.
To begin the second half of the jacket mold, I take two long strips and put them down the center. They overlap the other mold half about 1/2 inch (not too much farther or it will be difficult to separate the halves later). Finish the second half in the same manner as the first. Allow to dry completely before demolding, preferably overnight.
This is what the hinge mold looked like fresh out of the aluminum. The clay was not completely flat against the bottom, so there was some seepage - this is not a concern, as all of the overhang will be trimmed away later.
Also, the slimy looking bits are some cure inhibition, which I have no idea what caused it. In some cases, inhibition will eventually cure if left alone for a long time. Fortunately, they are pieces I will be cutting off anyway, so it doesn't matter in this case.
Clay removed. Now to carefully trim away the overhangs. Small craft or cuticle scissors work best.
After that, the molds are ready for casting.