Molding and casting simple rubber parts does not necessarily have to be done by a professional company. For small projects, an adventurous hobbyist can cast their own pieces using traditional mold making techniques. One of the best materials for making a mold suitable for casting rubber is rubber itself, since it does not stick together when cured. Start by making your own two-part silicone RTV rubber mold.
Create a template object or model out of clay, or choose an existing object from which to create a mold. Spray any soft clay with a clear sealing spray, such as Krylon Crystal Clear.
Build an open-top molding box out of wood or Legos. Make sure that the box can comfortably fit the object, with a clearance of at least 1/4 inch on each side. Make sure that the box can be easily disassembled. If using Legos, make the flat bottom out of modeling clay.
Mix up a small batch of silicone RTV rubber and catalyst, stirring until the mixture becomes a pale shade of the catalyst's color. Pour the silicone RTV mixture directly into the molding box, filling it halfway. Press the template object into the mixture until it reaches the halfway point. Do the same to a few wooden craft balls, placing them around the object. Let the silicone RTV mixture cure overnight, for at least twelve hours.
Mix up another batch of silicone RTV rubber and catalyst. Pour this mixture directly into the molding box, over the first layer of cured rubber. Make sure that your template object is covered completely. Let the silicone RTV cure overnight again.
Disassemble the molding box and separate the mold halves. Carefully remove your template object and the wooden craft balls. You should have a negative impression of each half of your object, plus notches made by the wooden craft balls that match the mold halves together. Fit the mold together and secure it with rubber bands. At the top or bottom of the mold, use the razor to carve a small channel through which to pour your casting rubber.
Always wear latex gloves when handling rubber mold materials, as they can irritate the skin. Always work with mold-making products in a well-ventilated area.