Evaporative sand casting is a fairly straightforward process. You carve a model out of a
low-melting point material. Afterward, solid rods of the same material are attached to the top and sides. The main rod attached to the top is called the sprue. The side rods are called the risers. The risers are bent upward, so all the tops of the risers and sprue extend about 3 inches past the top of the model. After the model is made, moist sand is packed tightly around the model, with only the tops of the sprue and the risers exposed. The sand is allowed to dry out. Molten metal is poured into the sprue. The heat of the metal evaporates away the model, filling the cavity with metal. The risers allow trapped air to escape, so the cavity becomes fully filled with molten metal with no void pockets. After the metal has cooled, the sand is shaken off. The risers and sprue are cut off, and the cast item is finished.
In the lost-wax process, the model, sprue and risers are made out of wax. For many objects with fine detail such as rings, the wax is given a coating of liquid ceramic-like clay. The reason it's coated with ceramic is because sand does not hold fine detail well. The model with the ceramic coating is packed with sand, and the sand is allowed to dry out. Molten metal is poured in the sprue. The metal melts away the wax, filling the cavity in the sand. Because the wax is evaporated away, this technique is called the lost-wax process.
Modern evaporative casting is very similar to the lost-wax process, except that you'd use polystyrene to make the initial model. The trade name for polystyrene is Styrofoam. You carve the model, sprue and risers out of polystyrene, and pack moist sand around it. You allow the sand to dry, and pour the molten metal onto the sprue. Sometimes you don't need a sprue and risers. The top of the model is exposed, and you pour the molten metal right in the top. The heat melts the plastic away, leaving metal in its place. Polystyrene blocks are inexpensive, so you don't have to worry about the cost to make a model.
Sand casting of evaporative models is by its very nature a one-time only process. The model itself is destroyed during the casting process. However, the models can be mass produced. Wax is molded into a reusable two part mold, which folds together like a suitcase. The mold is taken apart, leaving a wax model. The wax model is then packed in a sand mold. The permanent mold is re-used many times over. Polystyrene can be molded by the same method. Molten polystyrene is cast in a reusable mold. By making the models in a reusable mold first, models can be mass produced.