Collage of ductile iron castings, machine components, worm gears, spur gears, sand castings, investment castings and die castings

Castings Basics

Casting is a mechanical process whereby a molten metal or its alloy is poured in a mould and allowed to solidify. The object then gets the shape of the mould. Castings are manufactured in a single step from liquid metal without intermediate operations of mechanical working such as rolling or forging.

Castings are integral to the engineering industry, and have been in use for the last 6000 years. Casting is a mass production process that is used to manufacture complex shapes which are not very easy to attain otherwise. And the casting process is more economical and faster than other mechanical shape forming process.

A large number of metal components we use every day - from utensils to automobiles - are made by casting. Here are a few uses of castings:

  1. Auto ancillaries like cylinder and engine blocks, pistons and piston rings
  2.  Aviation and marine engine
  3.  Industrial tool beds and frames
  4. Numerous types of pipes and pipe fittings

The Casting Process

Casting begins with making a mould of the desired part in its geometric shape. There are two types of moulds used for metal casting process: expendable moulds and permanent moulds.  In expendable mould casting operations, the mould needs to be destroyed in order to remove the cast part. Since a new mould is required for each new casting, the production rates in expendable mould processes are often limited by the time to make the mould rather than the casting itself. These types of castings include sand casting (ductile iron casting / grey iron castings), investment castings, plastic castings, etc. It should be understood that expendable moulds do not necessarily mean inexpensive materials. Rather, it refers more to the moulds that have to be broken to remove the materials cast. There are many materials that can be used for expendable casting moulds like plastic, clay or metal moulds. These moulds are discarded after use. Sand casting is one of the most popular expendable moulds casting process.

In permanent mould casting processes, the mould is fabricated out of some durable material (usually metal / alloys) and can be used many times to make many castings. These castings hence can give higher production rates. Non expendable castings involves the use of long-lasting (more or less permanent) moulds which do not need to be broken in order to remove the cast material once it has set or cooled. These moulds are generally made of metal, but the method of performing the casting differs greatly from many expendable methods. One of the best known non expendable casting process is die casting. In die casting, molten metal under high pressure is forced into cavities of steel mould called 'dies'. Die casting is suitable for small to medium sized applications. One of the major advantages of die casting is the ability to use the die repeatedly. However, the die eventually deteriorates due to the high-pressure and high-speed injection of molten material.

The molten metal is poured into the mould cavity created by the utilization of a pattern. There are various types of patterns that are used in the casting process – split patterns, match-plate patterns or cope-and-drag patterns. Patterns define the external shape of the cast part. If the casting is to have internal surfaces, a core is required. A core is a full scale model of the interior surfaces of the part. It is inserted into the mould cavity before the molten metal is poured, so that the liquid metal will flow and solidify between the mould cavity and the core. This then forms the external and internal surfaces of the castings. Often, the core is made of sand that is compacted into the desired shape.

Once the molten liquid solidifies, the casting is complete. Depending upon the casting type used and the end use of the part, further treatment may be undertaken to improve the desired characteristics of the casting.

The advantages of castings are: 

  1. Castings can produce very complex parts with internal cavities and hollow sections. Since castings can create intricate objects, it reduces the number of other mechanical operations like machining, forging, etc.
  2. A carefully created die can allow creation of a single large machined component without assembling of smaller parts.
  3. Some metals / alloys can only be casted; they cannot be used otherwise.
  4. Most types of castings (sand casting, die casting, centrifugal castings, shell / permanent mould casting and investment casting (aka lost wax casting) is useful for mass production.
  5. Casting size can be used to make small (few hundred grams) to very large size parts (thousands of kilograms)
  6.  The casting process is economical, with very little wastage. All the extra metal in each casting can be re-melted and re-used
  7. One of the most important properties of metal castings is that it is isotropic. This means they have the same physical / mechanical properties along any direction.

Castings in India

There are three major players in the casting sector. China is the most dominant player with a share of about 40%, followed by India and U.S.A. with a share of about 5% each. While the industry was stagnant for the past few years, the casting industry in India is projected to grow at an average of about 7 - 8% in the coming years. For foundries and machined component manufacturers in India, the main demand is from European countries like UK, Germany and France. The growth in casting industry is expected to come from agriculture, infrastructure, automotive, railways, defence and oil & gas sectors. Needless to say, those who supply quality castings, machined components and other foundry products in India are poised to benefit the most.