The two part mold, a mold comprised of two interlocking pieces, is quite a simple mold to make. In general, a model is imbedded in modeling clay (plasticene or something comparable) up to a natural parting line. This line should follow the model's outermost contours, avoiding deep undercuts if possible. Determine this line carefully. After the figure is cast, you must be able to remove it and all subsequent castings without damaging the mold. Close inspection of commercial figures can prove helpful as even the best of castings will show some trace of flash.
Next, a box is assembled around this. A parting agent is then applied to all surfaces. The purpose of a release or parting agent is to keep uncured silicone rubber from sticking to anything. All types, especially silicone sealant, will adhere to almost any clean surface. In fact, the sealant is sometimes packaged as an "adhesive." What is needed is something to form a slippery barrier between the model and the rubber.
As it happens, a common household substance works well: petroleum jelly! It is however much too thick to be used as purchased. It cannot be brushed thin enough, leaving streaks in the mold's surface which later appear in your casting. It also fills in details in the model. These problems can be solved by thinning it with a common solvent. Paint thinner containing petroleum distillates works well, as does paint and varnish remover containing methylene glycol. Proportions mixing solvent and jelly are not critical. Thin it such that there is no streaking when it is applied to the model. As the solvent evaporates, it leaves behind a thin coating of petroleum jelly.
The box is now filled with the mold material. It will either be one-part silicone and plaster-of-Paris, or two-part silicone rubber. After that half has set, the box and clay are removed. The mold is inverted and boxed again, ready for the parting agent and mold-making components to form the second half.
The funnel and any necessary vents can be formed while making the mold or cut afterward. The funnel is where you pour the molten metal into the mold. Examine the model and decide how gravity will best work to fill the mold cavity. Again, a look at commercial castings will prove invaluable. As for the size of the funnel, it should hold at least enough metal to fill the mold. This is not a strict requirement, but it makes for easier casting.
It is usually necessary to "vent" a mold. In order to properly fill a mold, the metal must displace the air inside. The protrusions and appendages of models are good places for air pockets to form so a small outlet must be made. These outlets or "vents" are best made when the mold is itself made. This is easily done using pieces of wire. Imbed the wire in the clay, running it from the point you want to vent up and outward to the edge of the mold.
Avoid running vents through mold halves whenever possible. With complex shapes this is often unavoidable. when casting, the metal will fill the vent, harden, then break off as you remove the casting. You must then take the time to clear the vent. Should it be necessary to make such a vent, make it as straight as possible to ease the clearing process.
(RTV Silicone Rubber) , (Silicone Sealant / Plaster-of-Paris Mold) , (Table of Contents)