How to Make a Mold With RTV Silicone

Many products are cast out of silicone molds

Silicone is one of the most useful mold-making materials currently available. While more expensive than other materials such as plaster or fiberglass, its flexibility allows a nearly infinite range of items to be cast from it with no danger of the mold and cast locking together. RTV (Room Temperature Vulcanization) silicone is typically made of two parts, which are mixed, then allowed to cure. The most basic mold using RTV silicone is a box mold, good for small items with flat bases (chess pieces, bottles, etc.)

Things You'll Need

• Object to be molded
• Cardboard
• Hot glue
• Acrylic enamel spray
• Petroleum jelly
• Paint brush
• Mixing bowl
• Mixing stick
• Silicone and catalyst
• Vinyl gloves
• Scale

Instructions

1. Attach the object to be molded to a counter top, sheet of plastic or other non-porous surface with hot glue.

2. Seal the object with a few coats of acrylic enamel spray if it is made of a porous material such as clay or plaster. This will prevent the silicone from seeping into it and bonding.

3. Build a box around the object using cardboard and hot glue. The box should be a half-inch larger than the object. If it is less, the mold walls will not be strong enough; if it is more, you will waste silicone. Be sure to use enough hot glue to completely seal all sides, as well as the bottom to the working surface. Any gap will allow silicone to leak out.

4. Brush a thin coat of petroleum jelly to the inside of the cardboard box to act as a mold release. Otherwise the silicone will bond to the cardboard. This does not damage the mold as it will be on the outside, but it doesn't look very good over time as the cardboard begins to degrade.

5. Combine the silicone and catalyst in the mixing bucket according to the mix ratio on the container. Most molding silicones have a ratio of 10:1 silicone to catalyst by weight. Be as precise as you can.

   Silicone sticks to itself, so if you are unsure how much to use, err on the side of too little. You can always mix up a second batch if you don't have enough in the first.

6. Mix the silicone and catalyst thoroughly. Scrape the sides and bottom of the bucket several times as you mix. Molding silicones typically have a cure time of 3-12 hours, so there is no reason to do a rushed, sloppy job.

7. Hold the bucket as high over the cardboard mold as you can. Slowly drizzle the silicone into the mold, allowing any bubbles in the silicone to break in the thin drizzle. Pour the silicone into the lowest part of the mold and allow it to gradually fill. Fill the mold to at least half an inch above the top of the object.

8. Allow the silicone to cure. The exact time required will be printed on the silicone container.

9. Remove the cardboard and pull the original out of the mold.

Tips & Warnings

• Do not use latex gloves when working with silicone. Latex contains sulfur, which can inhibit the silicone's curing process.

References

• "The Prop Builder's Molding & Casting Handbook"; Thurston James; 1989
• Smooth-On: How to Make Molds & Castings