Silicone is used to produce durable, flexible molds that won't stick to the object being molded. While there are many silicone products made specifically for mold-making, you can achieve the same results with standard 100% (or outdoor) silicone caulk; it is basically the same product. Using the pour-box method will produce a high-quality silicone mold, as long as the model has no pronounced protrusions that would make it difficult to unmold.
Things You'll Need
Model to be molded
Small wood or plastic container to use for a pour box, just large enough to completely surround the model
Acrylic spray sealer (for wood pour box)
1 or more tubes 100% silicone caulk
Seal the inside of the pour box with spray acrylic sealer, if it is made of wood. Also seal the model if it is made from a porous material.
Place the model flat-side down in the pour box. Load the tube of caulk in the caulk gun, cut off the tip and break the seal with the scissors.
Squeeze the caulk into the pour box until the model is completely covered. Squeeze as slowly as you can to avoid introducing bubbles into the caulk. Open as many tubes of caulk as needed to fill the pour box.
Allow the caulk to reach full cured hardness, according to manufacturer's specifications (often up to several days).
Invert the pour box so the cured silicone drops out. Carefully remove the model from the silicone.
If your model has many tiny details, you may need to brush some of the liquid silicone into the details to make sure they are adequately covered. Then place the model in the pour box and cover it with caulk. If your model has protrusions or voids, you'll need to make a two-part mold instead. Make a ridge along the widest axis of the model with modeling clay, then paint the silicone on either side of the model with a paint brush. You may need to paint several layers to produce a thick enough model. Then remove the silicone from either side of the ridge.
Silicone-blend caulks will not cure firmly enough to use for molds. Always use 100% silicone caulk (often marketed as outdoor silicone caulk) for mold-making. Silicone caulks emit noxious vapors while curing. Use these products in areas with good ventilation.