The Basics of Sand Casting

There are many techniques that can be used in metal casting. One of those is sand casting. Sand mixed with clay and a bonding agent is used as the moulding material. Here we will take a look at some of the basics of sand casting.

The castings are typically made in specialised factories called foundries. The bonding agent used will depend a great deal on the solidity needed in the casting. The heavier the item is to be cast the more solid the casting will need to be. The sand is stored in a system of mould boxes called a flask. The actual moulds are created by compacting the sand around models of the item to be cast or by carving the shape of the item to be cast directly into the sand. The mould needs to be slightly larger than the required finished item as the metal contracts as it cools. How much bigger the mould needs to be is determined using a shrink allowance ruler.

There are two methods in sand casting; green sand and air set.

The green sand method makes use of wet sands to create the mould. The way the green sand is mixed will vary based on the type of metal that is going to be cast. Iron foundries tend to use a mix which uses silica instead of some of the other types of sand. They then need to be extremely diligent when it comes to dust extraction as excess moisture can explode during the pouring process and the inhalation of the resultant silica dust particles can cause silicosis in the foundry’s employees.

The air set method makes use of dry sand and a fast curing adhesive. This technique is also sometimes called no bake mould casting. While green sand casting uses a large variety of sands, air set casting will only use bank sand or lake sand. Lake sand is the preferred type as it is a synthetic sand and thus is more consistent in its composition. in the air set method, the sand is moulded using a casting flask which can be vibrated to ensure that the sand fills every cavity as compactly as possible.