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 integral to the engineering industry, and have been in use for the last 6000 years. Casting is a mass production process that involves pouring of a liquid into a mould of desired shape, which is then allowed to cool. They are used to make complex shapes which are not very easy to attain otherwise.

Castings are an integral part of our life. A large number of metal components we use every day - from utensils to automobiles - are made by casting. The advantages of castings are:

i. 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.

ii. A carefully created die can allow creation of a single large machined component without assembling of smaller parts.

iii. Some metals / alloys can only be casted; they cannot be used otherwise.

iv. 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.

v. Casting size can be used to make small (few hundred grams) to very large size parts (thousands of kilograms)

vi. The casting process is economical, with very little wastage. All the extra metal in each casting can be re-melted and re-used

vii. 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.

Types of Castings

There are two broad categories of castings: expendable and non-expendable casting. As the name implies, expendable castings are those where the mould is temporary. These types of castings include sand casting- (ductile iron casting / Grey Iron Castings), investment casting, plastic casting, 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 molds. These moulds are discarded after use. 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 dies 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.

Casting Process Simplified....

Here is a very simplified overview of the casting process:

1. Melt the metal in a furnace

2. Pour this melt into a mould or cavity which conforms to the shape of the desired component

3. Allow the molten metal to solidify in the mould

4. Take out the component from the mould, clean it and treat it further if needed

Castings are used in:

1. Auto ancillaries like cylinder and engine blocks, pistons and piston rings

2. Aviation and marine engines

3. Industrial tool beds and frames

4. Numerous types of pipes and pipe fittings

These are a very few applications of castings; they are useful in various other sectors.