Basic factors in casting

In casting, a solid is melted, heated to proper temperature, and sometimes treated to modify its chemical composition. The molten material, generally metal, is then poured into a cavity or mould, which contains it in proper shape during solidification. Thus, in a single step, simple or complex shapes can be made from any metal that can be melted. The resulting product can have virtually any configuration the designer desires. In addition, the resistance to working stresses can be optimized, directional properties can be controlled, and a pleasing appearance can be produced.

Cast parts range in size from a fraction of an inch and a fraction of an ounce (such as the individual teeth on a zipper), to over 30 feet (10 m) and many tons (such as the huge propellers and stern frames of ocean liners). Moreover, casting has marked advantages in the production of complex shapes, of parts having hollow sections or internal cavities, of parts that contain irregular curved surfaces (except those made from thin sheet metal), of very large parts, and of parts made from metals that are difficult to machine. Because of these obvious advantages, casting is one of the most important of the manufacturing processes.

Today, it is nearly impossible to design anything that cannot be cast by means of one or more of the available casting processes. However, as in all manufacturing techniques, the best results and economy are achieved if the designer understands the various options and tailors the design to use the most appropriate process in the most efficient manner. The various processes differ primarily in the mould material (whether sand, metal, or other material) and the pouring method (gravity, vacuum, low pressure, or high pressure). All of the processes share the requirement that the materials solidify in a manner that would maximize the properties, while simultaneously preventing potential defects, such as shrinkage voids, gas porosity, and trapped inclusions.

Six basic factors are involved in casting processes:

Mould Cavity. A mould cavity, having the desired shape and size, must be produced with due allowance for shrinkage of the solidifying metal. Any complexity of shape desired in the finished casting must exist in the cavity. Consequently, the mould material must be able to reproduce the desired detail and also must have a refractory character so that it will not be significantly affected by the molten metal that it must contain. Either a new mould must be prepared for each casting (expendable moulds), or the mould must be made from a material that can withstand being used for repeated castings. The latter type is known as permanent moulds. Since the permanent moulds are made of metal or graphite and are quite costly, their use is generally restricted to large production runs. The more economical expendable moulds are generally preferred for the production of smaller quantities.

Melting Process. A melting process must be capable of providing molten material not only at the proper temperature, but also in the desired quantity, with an acceptable quality, and within a reasonable cost.

Pouring Techniques. A pouring technique must be devised to introduce the molten metal into the mould. Provision should be made to permit the escape of all air or gases in the mould before pouring and those generated by the action of the hot metal entering the mould. The molten metal can then completely fill the cavity, producing a quality casting that is fully dense and free of defects.

Solidification Process. The solidification process should be properly designed and controlled. Provision must be made so that the mould will not cause too much restraint to the shrinkage that accompanies the cooling of solidified metal. Otherwise, the casting will crack when it is still hot and its strength is low. In addition, the design of the casting must be such that solidification and solidification shrinkage can occur without producing internal porosity or voids.

Mould Removal. It must be possible to remove the casting from the mould (i.e., mould removal). When the metal is poured into moulds that are broken apart and destroyed after each casting is made, there is no serious difficulty. However, in processes where permanent moulds are used, removal of the casting may present a major design problem.

Cleaning, Finishing and Inspection. After the casting is removed from the mould, various cleaning, finishing, and inspection operations may need to be performed. Extraneous material that is attached where the metal entered the cavity, excesses at mould parting lines, and mould material that is attached to the casting surface must all be removed.