This section provides information on the casting process for those who may not be familiar with it. This overview is not intended to be the definitive word on the casting process; but rather, a broad picture of the advantages of castings and the steps involved in making a casting.
A casting may be defined as a “metal object obtained by allowing molten metal to solidify in a mold”, the shape of the object being determined by the shape of the mold cavity.
Certain advantages are inherent in the metal casting process. These often form the basis for choosing casting over other shaping processes such as machining, forging, welding, stamping, rolling, extruding, etc. Some of the reasons for the success of the casting process are:
There are many more advantages to the metal-casting process; of course it is also true that conditions may exist where the casting process must give way to other methods of manufacture, when other processes may be more efficient. For example, machining procedures smooth surfaces and dimensional accuracy not obtainable in any other way; forging aids in developing the ultimate of fiber strength and toughness in steel; welding provides a convenient method of joining or fabricating wrought or cast products into more complex structures; and stamping produces lightweight sheet metal parts. Thus the engineer may select from a number of metal processing methods that one or combination, which is most suited to the needs of his work.
There are six basic steps in making sand castings:
The various steps in the production of castings are briefly summarized for the benefit of those who may be unfamiliar with foundries and the casting process.
The traditional method of obtaining the casting geometry is by sending blueprint drawings to the foundry. This is usually done during the request for quotation process. However, more and more customers and foundries are exchanging part geometry via the exchange of computer aided design files.
The pattern is a physical model of the casting used to make the mold.
The mold is made by packing some readily formed aggregate material, such as molding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled with metal to become the casting.
If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities.
Cores are forms, usually made of sand, which are placed into a mold cavity to form the interior surfaces of castings. Thus the void space between the core and mold-cavity surface is what eventually becomes the casting.
Molding consists of all operations necessary to prepare a mold for receiving molten metal. Molding usually involves placing a molding aggregate around a pattern held with a supporting frame, withdrawing the pattern to leave the mold cavity, setting the cores in the mold cavity and finishing and closing the mold.
The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the molds are filled.
Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. The casting is separated from the mold and transported to the cleaning department. Burned-on sand and scale are removed to improved the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Castings may be upgraded by welding or other procedures. Inspection of the casting for defects and general quality is performed.
Before shipment, further processing such as heat-treatment, surface treatment, additional inspection, or machining may be performed as required by the customer's specifications.