One of the best materials for making detailed molds is silicone RTV mold-making rubber. The RTV stands for “Room Temperature Vulcanizing,” which means that once you mix up the materials, you don’t need to put it in an oven to cure it.
There are a lot of ways that you can use silicone to make molds. The simplest by far is the “block mold.” In essence, you pour silicone into a box that contains your prototype. Once it cures into a solid rubber block you remove it from the box, remove the prototype from the mold, pour casting material into the mold, wait for it to cure, and then remove your part from the mold. Properly mixed and poured, detail reproduction with silicone rubber is usually flawless.
Since silicone rubber usually doesn’t stick to anything (the major exception being more silicone) it means you can cast parts without having to add any kind of mold-release agent that might fill in or otherwise obscure small details.
Know Your Terms
- Prototype: The original piece you’re trying to duplicate. Also called the “master” or “model.”
- Mold: The hole shaped from the prototype.
- Casting Material: The stuff you’ll fill the mold with in order to make the part.
- Part: The copy that comes out of the mold.
Build a Block
The box needs to be big enough to get about ½” of material around the piece. Scrap cardboard and hot glue are perfect for mold boxes. It’s vital to make sure to glue all of the seams well, both inside and out, to keep the liquid silicone from leaking. Make sure your prototype is free of any flaws, including fingerprints, as these will show up in every piece.
Put a dab of glue on the bottom of the prototype and center it in the box. This keeps the prototype from shifting or floating away as the silicone is poured. Mount it on a piece of clay to give the part some extra material that can be cut off later.
Banish the Bubbles
Mix up a batch of silicone in accordance with the manufacturer’s instructions. Note: Most silicone mixtures are determined by weight, not volume.
When you mix and pour silicone rubber, the biggest enemies are air bubbles. Air bubbles trapped in the rubber will become weak points that will possibly tear open and cause the mold to fail. Bubbles trapped against the surface of the prototype will become lumps of extra casting material in the final parts.
Start by pouring the liquid rubber into a corner of the mold so that it fills up and flows across the surface of the prototype. This minimizes the number of bubbles that get captured against the surface. Lift your container as you pour to draw the silicone into a thin strand, which aids in the rupturing of bubbles created during mixing.
If you have access to a vacuum chamber, you can evacuate bubbles from the liquid rubber after you mix it. If you don’t, just use a type of silicone with a longer cure time in order to give the bubbles more time to float to the surface.
Once the rubber has cured solid, tear away the cardboard mold box around the part and gently peel the rubber off of the prototype. Now you can use the mold to cast parts in a variety of materials.
Whenever you cast urethane resin parts in a silicone mold, there will be a tiny bit of oily residue from the silicone that is left on the parts. In order to make the parts ready for painting, the residue has to be washed off with warm, soapy water.