Recipe 17.2 : How to Make Silicone Prosthetics - Part 2
- Koji Ohmura
- Jan 13
- 3 min read
In Recipe Part 1, I explained how to design prosthetics on a flat board. In Part 2, I show you how to mold it. Did you miss Part 1? If so, please click the button below.
3.Mold Making Session
Materials you need...
Crystal Clear
Kleen Clay
Pasta Maker
Sculpting Tools
Hot Glue Gun
Wooden Sticks
Platinum Cure Molding Silicone (Smooth Sil 945 or 940)
Evacuator
Non Latex Gloves
Chip Brush
Air hose
Knife
99% Alcohol
Material links are here.
Lesson Video
Flashing
In order to make a proper flat mold, you will need proper preparation. I normally seal the sculpture with Crystal Clear first. This will protect the sculpture from minor scratches when you mold it, and it makes it easier to clean the mold later.
Spray briefly all over the sculpture. Let it dry.
1 layer of Crystal Clear While you are waiting for it to dry, let's prepare Kleen Clay. It is an oil-based, sulfur-free clay. You need it in a thin, even thickness. I always use a pasta maker for it.
The pasta maker comes in handy. Once you get a strip of Kleen Clay, trim both sides at a 45-degree angle. Place it around the sculpture, keeping 1/8 inch from the sculpture. This is called "flashing" and is made for silicone to escape while casting prosthetics.
Avoid undercuts. Spray another layer of Crystal Clear.
Flashing all around the sculptures.
Creating Dam Walls
Next step is making dam walls out of mixing sticks. I recommend wooden sticks. You can recycle after using them.
You can glue them with hot glue onto the board. Keep a little space from the flashing. Then, make sure there is no holes in between sticks and board. It should be no leakage.
Wooden sticks with hot glue.
Mixing Silicone
For mold silicone material, I used Smooth-Sil 945 from Smooth-On. It is platinum cure silicone. Please do NOT use tin cure silicone for this mold making. Since we are creating platinum cure prosthetics, we need a platinum cure mold. Otherwise, the finished product stays uncured and slimy.
You can use other hardness for this flat mold. I like Smooth-Sil 945 or 940 hardness. Please make sure there is no unmixed silicone. It is a good idea to transfer the mixed silicone to another clean cup before pouring it.
Mix well together. (Optional) If you have a vacuum chamber, it is a good idea to use it to get rid of microbubbles. When bubbles come out, the silicone mix rises up, so use a bigger bucket for this. I leave it in the chamber for 1 minute. Do not wear latex gloves, as latex will inhibit platinum silicone from curing. Vinyl gloves or nitrile gloves are recommended. If you do not have a vacuum chamber, pour the silicone from a high point slowly.
Silicone cups in a vacuum chamber.
Pouring Silicone
Ideally, pouring from one corner of the dam is the best way for an even, smooth flow. However, considering Smooth-Sil 945's pot life (working time before it gets thicker), which is 25 minutes, I pour it everywhere. Then, I use a chip brush to brush it on all surfaces. By brushing it on, ensure all details are brushed.
Pouring from a high point. Use an air compressor and air hose to pop bubbles. This will help the silicone to go into the textures of the sculpture.
High air pressure will help silicone capture all details. Pour the rest of the silicone over the layer. Cover the entire sculpture and check that there is no leakage. If you see it leaking, use clay to fill the gap. If it does not stop, super glue and zip kicker might help. Silicone will set up after 6 hours.
Opening Mold
After the silicone is completely cured, remove the dam walls. You can use them again in the future.
Slowly lift the silicone mold from the sculpture. Even though silicone is flexible, it is still breakable, so do not pull it out harshly. If the sculpture is still in good shape, you can make mold copies.
Fresh mold Trim the edge of the mold where it warps. Normally, it is near the dam walls.
Wash the mold with 99% alcohol. This will remove clay chunks and crystal clear spray.
Trim the edges and wash it with 99% alcohol.
The mold-making is done! The next step is finally creating prosthetics. Please click the button below for the next page.
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