Metal Casting Zone Info Blog

mike1942f:

A hole is created in a heat proof material and molten metal is poured in. The hole, besides having the shape of the result, has to have sprues to guide the metal to all parts of the hole and vents to let the air out.
The hole can be made in specially treated fine sand which is packed around a solid model, usually wood, so tightly that it can be lifted off. The parts of the sand mold are fitted together, perhaps with cores to make holes in the casting and the sprues and vents are carefully cut in the sand and the metal is poured in to the sand mold which is destroyed to get the part out.
The hole can be made by making a model in wax with added prues and vents, then pouring a special plaster like compound called investment around it. The cast investment is heated gradually, first to drain the wax out, then to burn out the mold, then to get it to casting temp. The molten metal is poured in and the investment washed off.
The hole can also be made with a wax model that is coated with a thin slurry of silica wash, then dusted with silica flour, dried and this is repeated until a strong layer has been built up. It is then heated to drain the wax and further heated to burn it out, then molten metal is poured in. Unlike investment, which is weak and needs to be contained, silica molds look like a fluffed out version of the model and are strong.
Large bronze statues are commonly made with a thin wax layer over clay, and covered with clay. The clay is heated to melt the wax out, then further heated and the metal poured into the narrow space so that only a thin shell if metal is needed instead of a solid body. Often they are made in sections and welded (braised) together and then cleaned up.
Almost all casting involves considerable cleanup work to cut off the sprues and vents and clean the seams and rough spots.