Things You'll Need
Hot wire foam cutter
Clear liquid sealer
Mold release compound
Empty jelly jar
Polystyrene is a useful material for model builders and other crafters. If you wish to make a mold for use in producing several copies of an item, polystyrene is an easy material to work with. There are two basic methods for creating a polystyrene mold. You can carve it by hand, or you can liquefy it and pour it into a mold box.
Shape a polystyrene foam block with a hot wire foam cutter. Fine details can be added with any wood carving tools or a carving knife. Carve the shape in the negative form so you can use the foam shape directly as a mold.
Brush on a layer of clear sealer over the mold. Allow to dry for at least four hours. You may wish to apply several layers. Do not use an oil-based sealer as it may melt the polystyrene.
Brush on mold release compound before you pour your plaster or other model material into the mold.
Set up your mold box, which can be any size needed for your project. The "box" can be a glass cake form, a metal pan or a custom box you make from wood. Just make sure any seams are completely sealed.
Cover the mold object that you wish to copy with mold release compound. Place the object in the mold box so its base is even with the top of the box. You may suspend it with sticks taped to its base or you may place it on top of something like a small block of wood, which will create a shape you can cut off the finished object later.
Pour about 1/2 cup of acetone into an empty glass jelly jar.
Break a polystyrene foam block into pieces small enough to fit inside the jelly jar. Place them in the acetone one at a time, being careful not to get acetone on your skin. The polystyrene should melt into a gluey, liquid substance. If it is not liquid enough, pour in more acetone.
Pour the liquid polystyrene into the mold box. Fill the mold to the top. Let the mold set for 24 hours. The acetone should evaporate and the polystyrene set in about 12 hours, but wait a full day to make sure it fully cures.
Carefully remove your object from the mold. If you want a two-part mold, carefully cut the finished mold in half along the longest seam of your object with a sharp knife or thin saw blade. Seal your finished mold before use with clear sealant.
Work in a well-ventilated space when using acetone and use protective eyewear.