Useful Articles 

The Benefits of Casting as a Manufacturing Process

Casting has been defined as the conversion of molten metal to a pre-engineered shape or form. Since there are about a dozen different commercial casting processes, each with special benefits, the design and manufacturing engineer has many options and decisions to make.

Unlike stamping or drawn shapes, casting allows the designer to put metal where it is most needed. Parts can be streamlined and contoured for aesthetic and cosmetic appeal. Welded fabrications can be eliminated by casting combined sections. Wall thicknesses can be varied within reasonable limits of good casting design. Pads and bosses can be added for mounting supplemental equipment. Special castable alloys like gray iron provide superior dampening of mechanical noise and vibration. An economical, repetitive process, casting turns out thousands of part within specific dimensional tolerances. The size of component parts is limited only by the producer's facilities to mold, melt, and pour. Casting sizes range from fractions of an ounce to hundreds and even thousands of pounds.

With today's technologies, parts are being cast which could not be made by any other method. The practical use of casting technology has commercial cost efficiencies based on man's ability to develop molding and die materials capable of handling the extreme heat of molten metal. Over the past decades new innovations have been made in castable alloys and techniques. Even now, the search continues for better processes and refractory materials.

SAND CASTING

Sand, one of the oldest refractories known to man, offers significant benefits, particularly in low cost and versatility. The biggest cost to sand comes in hauling and handling its movement. Its versatility results from the wide variety of bonding agents available to hold the sand in mold form.

"Green sand," which takes its name from its high moisture content rather than its color, provides an outstanding molding media for low cost, high production, automatic molding. Since about seventy-five percent of all sand casting defects are sand related, controlling the properties of green sand is vitally important to a profitable operation. The sand requires constant monitoring to produce consistent quality parts. Modern foundries will have automatic sand control and monitoring equipment; in addition, someone will have the responsibility of checking sand strength and molding properties on a periodic basis every day.
The use of chemical bonding materials, most of which are activated with a combining catalyst, is more common in a job shop type foundry. The broad variety of air-set and chemical catalytic binding agents are classed as "dry sand" techniques. These processes include the CO2 process, which uses carbon dioxide gas to activate a sodium silicate (water glass) binder coating the sand. More dry sand processes are being automated, but few rival the economy of a well controlled green sand operation.

Regardless of the sand used, patterns are constructed in wood and duplicated or machined from plastic and metal.

To make the molding process easier, patterns are often made in matching halves. In short run applications, a mold is formed by ramming or squeezing sand against each pattern half. The job of molding becomes easier when the patterns are mounted on a wooden board or metal plate. Two half molds, stacked one on top of another so that the cavities in each mold are precisely aligned, constitute a full mold. Molds can also be booked vertically. Every mold has a cavity to form the part being cast, loose pieces in the mold to form undercuts or hollow sections in the part, and a channel or passageway for the metal to enter the mold cavity. In foundry parlance, this channel is called the "gating system"; each gating system has a sprue, a runner or runners, and one or more gates. Depending on the nature of the part and the metal being poured, separate reservoirs attached to the mold cavity or the gating system may be necessary to supply molten metal to the casting as it solidifies. These reservoirs are called "risers" or "headers."

There are basically three prerequisites to making quality castings:

1. Consistent, disciplined, sand control.
2. Properly designed "gating," the term used to describe the channeling of metal to the mold cavity.
3. Consistently accurate metallurgical practices in melting and pouring (i.e., temperature, deoxidation, raw material selection).

Any metal which can be air melted can be sand cast, and the sizes of cast components range from a few ounces to thousands of pounds. Where flasks would be impractical for very large castings, molding and pouring are done in pits. These pits are constructed in the foundry floor.

GREEN SAND MOLDING

Green sand molding utilizes a mold made of compressed or compacted moist sand. The term "green" denotes the presence of moisture in the molding sand, and indicates that the mold is not baked or dried. The mold material consists of silica sand, mixed with a suitable bonding agent like clay, and moisture. To produce the mold, a flask is placed over the pattern to produce a cavity representing one half of the casting. Compaction is achieved by either jolting or squeezing the mold. The other half of the mold is produced in like manner and the two flasks are positioned together to form the complete mold.
If the casting has hollow sections, a core consisting of sand, hardened by baking or through chemicals, is used. If extra support for the core is required, chaplets or spacers are positioned to maintain the required dimensions. These will fuse with the molten metal when the casting is poured. Green sand is the best known of all the sand casting molding methods, as the molds may be poured without further conditioning. This type of molding is most adaptable to light, bench molding for medium-sized castings or for use with production molding machines.

Advantages
Most metals can be cast by this method. Pattern costs and material costs are relatively low. The method is adaptable to large or small quantities.

Disadvantages
There are practical limits to complexity of design. Machining is often required to achieve the finished product. Dimensional accuracy cannot be controlled as well as with other molding processes, although good standards are possible with quality pattern equipment, modern process controls, and high-density molding.

Back to top  |  Back to main article listing