Things You'll Need
Legos or molding box
Silicone RTV rubber
Pencil or marker
Razor blade or utility knife
Silicone molds are most often used in projects that require resin or other plastic casting, and can be seen in the creation of toys, collectible figurines, props, model parts and other small plastic casting projects. Silicone rubber is usually preferred because it can be used with many materials and it does not stick to itself or other objects, negating the need for a mold release agent or a separating wall during the molding process. The most common type of silicone mold is the two-part mold, which creates a seamless end product.
Build a molding box for the model object you would like to make copies from. For smaller objects, make your box walls from Legos, and roll out a flat piece of modeling clay for the bottom of the box. For larger objects, build the box walls and bottom from sturdy cardboard or wood. Make sure that the box gives your model a clearance of at least 1/4 inch on all sides.
Mix together half a batch of silicone RTV rubber and catalyst, following the packaging directions for the brand of rubber you have. Make sure that the catalyst is completely blended into the rubber, creating a pale version of the catalyst's color.
Mark the halfway point on your molding box with a pencil or marker. Pour the batch of silicone rubber into the molding box, and stop at this halfway mark.
Press the model object into the silicone rubber until it is half covered and does not touch the sides of the molding box. Let this half of the mold cure, or dry, overnight or for 12 hours.
Mix together the rest of the silicone RTV rubber, and pour it into the molding box, covering the model object completely. Let the rubber cure overnight.
Take the molding box apart and separate the mold halves, removing the model object. Fit the mold halves back together and secure them with rubber bands.
Carve a pouring hole in the top of the mold at the joining line with a razor blade or utility knife. Cut the hole deep enough to penetrate through the rubber and reach the inside cavity.