Recipe 4 : Molding Prosthetic Sculpture
- Koji Ohmura
- Sep 16
- 5 min read
Updated: Nov 2
When you use a waffle maker, you see two parts: top and bottom. With these two parts together, it is called a mold. And you pour waffle mix into the waffle mold. Yummy! This session is very similar to the waffle mold. Instead of a waffle, we make a prosthetic!
Recipe 4 features how to make a negative mold. Then, you can have your waffle mold! Just kidding, you will have a werewolf nose mold.
Materials you need...
Krylon Crystal Clear
Lacquer Thinner
Q-tip
Round Nose Router Bit (Keys)
Chavant Sculptex (Kleen Clay)
Sculpting Tools
Power Drill
Pasta Maker
Krylon Dulling Spray
Smooth On Sonite Wax
WED Clay
Vaseline
Ultracal 30
Water
Gloves
Mixing Bowl
Rasp
99% Alcohol
Chip Brush
Material links are here.
Lesson Video
Mold Prep
First, you will need to seal the prosthetic sculpture from Recipe 3. One layer of Krylon Crystal Clear is enough for now. It is just a protection at this moment.
Then, pour a small amount of lacquer thinner into a small cup. Use a Q-tip to clean the outside of the prosthetic clay sculpture. This will clean both extra clay and crystal clear from the last step. (Do not pass the edge of the sculpture. It will take away the skin texture you sculpted.)
Clean right by the edge. Now, you need "keys." These will be your registration marks when you close this mold. Without these keys, the mold will not fit well. Set a Round Nose Router Bit on a power drill. Then, at the slowest speed, drill a half-ball shape on the mold surface. You will need three keys. Clean the keys.
Carefully! Drill the key in slowly.
Casing Mold
Now, the mold is ready for casing. You will need a pasta maker and Kleen Clay. I used Sculptex from Chavant in this video.
Take a small amount of Kleen Clay and insert it into the pasta maker. If you do not have a pasta maker, you can just flatten it by hand. Remove any air pockets if there are any.
Create a strip of flattened Kleen Clay. Cut one side of the strip at a 45-degree angle.
Place the angled side next to the edge of the sculpture. Keep a distance of 1/8 inch. Continue this all around the sculpture. Do not cover the keys. These are called "Cutting Edge" and "Flashing." Cutting Edge will help the prosthetic have a very smooth blending edge, and Flashing will help the prosthetic material escape when the mold gets extra material.
Cutting Edge is 1/8 inch long. If you see undercuts, fill them with Kleen Clay. If you look from the top and side and see a dip going in, that is an undercut. If you do not fill the undercuts, the finished mold will not open.
Undercut by the nose 
Fill this undercut with Kleen Clay. Once all the areas are covered, make a snake with Kleen Clay. Place it over the edge of the sculpture gently. Also, cover the keys with a ball of Kleen Clay.
Spray 2 layers of Crystal Clear. Dry each layer. Do not use a hair dryer. It will soften your sculpture.
Spray 1 layer of Dulling Spray. Let it dry.
Remove the snake and balls. Clean the keys with lacquer thinner.
Paint Sonite Wax on the keys. Dulling Spray & Sonite Wax will the Mold Release.
"Clay Snake" will keep the cutting edge clean.
Clay Wall
In this step, we are making a clay wall. It is like a dam.
Cut WED Clay into 1-inch thickness. Place it next to the positive mold. Place it all around. Check that there are no gaps in between. Since we are pouring plaster on it, you do not want it to leak.
Check that the top of the wall is higher than the top of the prosthetic sculpture. if not, add more WED Clay on top of the dam wall.
Make sure there is no leakage.
Molding
Pour a small amount of water into a bowl. (Paint Vaseline on the bottom of the bowl)
Sprinkle Ultracal 30 into the water until the surface looks like a dried lake.
Mix it with your hand. Ensure no chunks are left in the mixture.
Dried lake surface Brush a thin layer of plaster on the positive mold.
Wait 10-15 minutes, and check if it is dry but not completely cured.
Mix another batch of Ultracal 30 and splash it on the first layer. Do not touch the first layer. It is very fragile yet.
Once the first layer is covered, gently brush over the splashed layer and make it smooth. Wait another 10 minutes. Let it dry.
Splashing with a chip brush. Mix a third batch of Ultracal 30 and splash it on the second layer.
Dip small shapes of Burlap in the Ultracal 30. Place them on the mold gently.
Dip burlap in Ultracal 30. As you add more layers, push bubbles out of each layer. Create 4-5 layers of them.
Pour the leftover into the dam until the top of the wall.
Cover the mold with a plastic bag.
Opening Mold
Wait for 2 - 3 hours. It gets hot, and it cools down.
Take WED Clay wall out. If it is still soft, you can reuse the WED Clay later.
You can recycle this wall! Sand off sharp edges with a Shinto rasp or regular rasp.
Before opening the mold, draw a registration mark so you will know where the mold meets.
Use a flathead screwdriver, and open the mold. When you open it, use a twisting motion so it does not crack the mold.
Twisting a screwdriver clockwise. Once it opens, use a wooden tool to take off clay from the mold. Do not use a metal tool to clean the mold surface. Keep the NSP Clay. This will tell you how much gelatin you need next time.
Clean the residue with lacquer thinner. Also, 99% alcohol works for cleaning.
Clean the mold completely. After it is clean, brush on a furniture paste wax and buff it off. This will fill micro bubbles. Do this 2 times.
Put a positive mold and a negative mold together. Tie them together with a mold strap for storing.
Making a mold is very complicated. However, it is so calming if you are not rushed. It is almost like therapy. It is definitely my favorite process during prosthetic making. There are many great artists who are mold makers and work in FX labs.
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