The Probert Encyclopaedia of Science & Technology

CASTING

Casting (founding) is the process of producing solid objects by pouring molten material into a shaped mould and allowing it to cool.

Casting is used to shape such materials as glass and plastics, as well as metals and alloys. The casting of metals has been practiced for more than 6,000 years, using first copper and bronze, then iron.

The traditional method of casting metal is sand casting. The foundry floor was composed for several feet deep of a loamy sand, in which deep pits were some times sunk to buty large moulds. A wooden pattern of the object to be cast having been made, was pressed firmly down into this loamy sand, the sand being shovelled up all around, level with the top of the pattern and well rammed down. The pattern was then lifted out of the sand, and any small pieces of sand which may have fallen into the mould were carefully blown away, and some finely-powdered charcoal sifted over the surface. The molten metal was then poured into the mould until it was full. The whole was then covered with sand to keep the air from it while it cools. An open horizontal bed of sand was sufficient for casting many articles, but with articles of a more complex form and not too large, a frame or box, called a flask was generally employed to hold together the sand used in the casting, the number of flasks varying according to the form and parts of thee mould.

In ordinary operations the pattern was laid on a board known as the turn-over board, and the flask placed over it, the sand being carefully rammed into the flask until it was full. Another board, known as the bottom-board, was then laid upon it. The flask was then turned over, the first or turn-over board taken off, the one side of the pattern uncovered, a fine facing of sand spread upon the surface to prevent adhesion, after which a second flask, called the cope, sometimes made with crossbars to strengthen it and help to hold the sand, was placed upon it and sand carefully rammed in. The cope or second flask was then lifted off, the sand which it contains carrying the impression of the upper side of the pattern; the pattern in the lower part of the flask, or drag, was then carefully drawn out, and any injuries which the mould receives during the operation repaired. Holes or passages were then cut into the sand for pouring in the metal, all loose sand carefully removed, the cope replaced and secured to the drag by clamps. The mould was then ready for the molten metal. In pouring, the metal was generally run through two or three different passages at the same time to prevent it losing fluidity by cooling. It was only in lighter castings that sand, of the proper degree of dryness, porosity, and adhesiveness, was used.

In heavy castings the mould was usually made of loam, which is more adhesive, and in complicated articles the making of the mould was often a difficult process. Small articles of simple form and of easily-fusible alloys, such as bullets, printing types, etc, were often cast in metal moulds. Articles of sculpture were usually cast in plaster of Paris, which, when mixed with water, runs into the finest lines of a mould and takes a most exact impression. The variety of articles made by casting was very great: boilers, cisterns, cylinders, pumps, railings, grates, cannon, cooking-utensils, and many objects of decorative art.

Permanent metal moulds called dies are also used for casting, in particular, small items in mass-production processes where molten metal is injected under pressure into cooled dies. Continuous casting is a method of shaping bars and slabs that involves pouring molten metal into a hollow, water-cooled mould of the desired cross section.
Research Casting