Castings

Quick Read

Casting is a simple, inexpensive and versatile way of forming aluminum into a wide array of products. Such items as power transmissions and car engines and the cap atop the Washington Monument were all produced through the aluminum casting process. Most castings, especially large aluminum products, are usually made in sand molds.

Take-Away Facts

  • Casting must include a part-removal design
    Casting molds must be designed to accommodate each stage of the process. For part removal, a slight taper (known as draft) must be used on surfaces perpendicular to the parting line so the pattern can be removed from the mold.
  • Casting parts with cavities
    To produce cavities within castings (such as for engine blocks and cylinder heads used in cars), negative forms are used to make cores. Casts of this nature are usually produced in sand molds. Cores are inserted into the casting box after the pattern is removed.
  • Casting for light weight and strength
    Aluminum’s properties of light weight and strength bring fundamental advantages when cast into parts. One common application of die cast aluminum is thin-walled enclosures with ribs and bosses on the interior to maximize strength.
  • Casting in the early history of aluminum
    The first commercial aluminum products were castings such as decorative parts and cookware. Though produced through a centuries-old process, these products were considered new and unique.

Aluminum Casting 101

The process of casting aluminum

Casting is the original and most widely used method of forming aluminum into products. Technical advances have been made, but the principle remains the same: Molten aluminum is poured into a mold to duplicate a desired pattern. The three most important methods are die casting, permanent mold casting and sand casting.

Die casting

The die casting process forces molten aluminum into a steel die (mold) under pressure. This manufacturing technique is normally used for high-volume production. Precisely formed aluminum parts requiring a minimum of machining and finishing can be produced through this casting method.

Permanent mold casting

Permanent mold casting involves molds and cores of steel or other metal. Molten aluminum is usually poured into the mold, although a vacuum is sometimes applied. Permanent mold castings can be made stronger than either die or sand castings. Semi-permanent mold casting techniques are used when permanent cores would be impossible to remove from the finished part.

Sand casting

The most versatile method for producing aluminum products is sand casting. The process starts with a pattern that is a replica of the finished casting. Virtually any pattern can be pressed into a fine sand mixture to form the mold into which the aluminum is poured. The pattern is slightly larger than the part to be made, to allow for aluminum shrinkage during solidification and cooling. As compared to die and permanent mold casting, sand casting is slow process but usually more economical for small quantities, intricate designs or when a very large casting is required.

Casting Applications

Widespread use in the automotive industry and homes

The automotive industry is the largest market for aluminum casting. Cast products make up more than half of the aluminum used in cars. Cast aluminum transmission housings and pistons have been commonly used in cars and trucks since the early 1900s. Parts of small appliances, hand tools, lawnmowers and other machinery are produced from thousands of different unique aluminum casting shapes. The casting product most often used by consumers is cookware, the first aluminum product that was made available for everyday use.

Aluminum-intensive cars save 44 million tons of CO2 emissions.

Independent studies have confirmed that aluminum has a 20 percent smaller life cycle CO2 footprint than steel. And compared with today’s steel cars, a fleet of aluminum vehicles saves the equivalent of 44 million tons of CO2 emissions.