Plastic or urethane resin casting requires a mold material that is tough and resilient. Plastic is a difficult material that can often pull the moisture from a mold, causing the mold to be more likely to crack and shortening its length of usability. The best mold material to combat this kind of casting problem is silicone RTV, or room-temperature-vulcanizing, rubber. Silicone RTV rubber creates flexible, tear-resistant molds that are capable of producing fine detail and can withstand high temperatures. To create your own silicone rubber mold for plastic casting, complete this 3-day process.
On the First Day
Prepare your template object and work space. Cover a flat area with plastic trash bags to avoid mess and spray the plastic with a mold release agent.
Build a molding box, a frame that holds the mold in place, that is bigger than the size of your template object. You can make your frame out of wood or metal.
Measure out a small amount of your silicone RTV rubber using a reliable scale, and mix it, following the directions on the package.
Add your catalyst. The ratio of rubber to catalyst should be about 10:1. Stir the mixture well, and do not to leave too many gas bubbles.
Apply the rubber mixture to your template object to make an impression coat. You can pour the rubber over the object with a ladle, or paint it on. Make sure that the whole object is coated. Let the object stand for an hour.
Apply more of the silicone RTV rubber mixture to your template object the same way you did before. The rubber should stick better this time. Let dry for an hour and repeat this process 3 times, then allow the object to dry overnight.
On the Second Day
Mix up another batch of silicone RTV rubber, following the same process you used on the previous day.
Apply a fresh coat of rubber mixture to the template object. While the object is still wet, layer small pieces of cheesecloth over it. Make sure that the rubber soaks into the cheesecloth. Keep adding cheesecloth until the entire object is covered in at least two layers.
Apply another coat of silicone rubber to the template object. Let this coat dry overnight.
On the Third Day
Create a mold reinforcement. Coat the inside of your molding box with dishwashing liquid or petroleum jelly, and fill the box with wet plaster.
Press your template object into the wet plaster, allowing for some space at the bottom. Let the plaster set until it begins to get warm, then create keying or registration depressions around the object.
Let the plaster cool off. Coat the surface with dishwashing liquid or petroleum jelly and pour plaster over the top of the template object. Do not move on to the next step until the wet plaster has hardened and cooled off.
Remove the plaster from the molding box. The two halves should come apart easily, releasing your rubber mold.
Make a slit down the backside of the silicone RTV rubber mold with a sharp razor blade, creating a seam. Carefully pull the template object out of the mold.
Cut the bottom of the plaster parts of the mold on a band saw. You will want to expose a hole to pour your plastic casting material into. Be sure that the spout part of the rubber mold will protrude slightly through this hole.
Fit your rubber mold back into the plaster mold and tie it shut with some rope or twine. You should now have a spout in the bottom of the mold perfect for casting.
If your template object is porous, you may need to use a sealer to prevent the silicone rubber from soaking in. Keying depressions can be made by scooping shallow round sections of plaster out of the mold. Be sure that your molding box has sides high enough to allow for the second plaster pouring, or create a second "frame" that is the same size as the box. You can create a spout for the rubber mold by adding modeling clay to the bottom of the template object before you start applying liquid rubber.
Wear gloves and safety goggles at all times. Do not let the wet rubber touch your skin or eyes.