Casting is the original and most widely used method of forming aluminium into products. Technical advances have been made, but the principle remains the same:
Molten aluminium is poured into a mould to duplicate the desired pattern. The three most important methods are die casting, permanent mould casting and sand casting.
Die casting
The die casting process forces molten aluminium into a steel die (mould) under pressure. This manufacturing technique is normally used for high-volume production. Precisely formed aluminium parts requiring a minimum of machining and finishing can be produced through this casting method.
Permanent mould casting
Permanent mould casting involves moulds and cores of steel or other metal. Molten aluminium is usually poured into the mould, although a vacuum is sometimes applied. Permanent mould castings can be made stronger than either die or sand castings. Semi-permanent mould casting techniques are used when permanent cores would be impossible to remove from the finished part.
Sand casting
The most versatile method for producing aluminium 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 mould into which the aluminium is poured. The pattern is slightly larger than the part to be made, to allow for aluminium shrinkage during solidification and cooling. As compared to die and permanent mould 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 aluminium casting. Cast products make up more than half of the aluminium used in cars. Cast aluminium 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 aluminium casting shapes. The casting product most often used by consumers is cookware, the first aluminium product that was made available for everyday use.
Take-Away Facts
- Casting must include a part-removal design
Casting moulds 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 mould. - 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 moulds. Cores are inserted into the casting box after the pattern is removed. - Casting for light weight and strength
Aluminium’s properties of light weight and strength bring fundamental advantages when casting into parts. One common application of die cast aluminium is thin-walled enclosures with ribs and bosses on the interior to maximise strength. - Casting in the early history of aluminium
The first commercial aluminium products were castings such as decorative parts and cookware. Though produced through a centuries-old process, these products were considered new and unique.