- Expendable mould casting: In this kind of casting process, the mould cavity is created by packing a refractory material around a pattern. Such moulds are called one casting moulds. The moulding material could be sand or any other refractory material. The main drawback of sand moulding process is that high dimensional accuracy and surface finish are not achievable for the modern industry standards. This can be avoided if instead of sand, use of slurry is made. The pattern used in this kind of casting is permanent pattern which is made of wood, metal or plastic.
- Permanent mould casting: Unlike the above mentioned casting process, the mould in this casting process is reused for multiple casting solidifications, usually thousands of castings. We can use this process to make small and medium sized castings from non-ferrous alloys, with smooth surface and high dimensional accuracy. As the cost of pattern is high, we usually make use of this casting process for mass production setups.
- Semi permanent mould casting: The moulds used in this process are not as durable as the permanent moulds, so it is not conducive for mass production setups. The moulds are produced from high refractory materials, like graphite based materials.
Types of moulds:
- Green sand moulds: This kind of mould is composed of a mixture of sand (silica sand), clay (binder) and water. The term 'green' signifies the fact that the mould is left in a damp condition. It is the easiest mould that can be used, but as the mould is in damp condition, it is therefore weak and cannot be used for a longer time period. This makes such moulds suitable only for small and medium sized castings.
- Dry sand moulds: These are also green sand moulds, but with a considerable difference, i.e. there are two additives used in the sand used in dry sand moulds. The sand contains 1-2% cereal flour and 1-2% pitch. In addition to this, the moulds are baked in oven at 110 to 260oC for multiple hours. The additives increase the hot strength of the mould with the help of evaporation of water and oxidation of the pitch. Dry sand moulds can be used for large sized castings and provide a better surface finish. This kind of mould reduces the casting defects like gas holes, but due to greater strength of the mould, tearing may occur in the materials.
- Loam sand moulds: Such moulds consist of loam sands, which comprise of fine sand with finely ground refractories, clay, graphite and fibrous materials. This kind of sand has a high value of clay content in comparison to ordinary moulding sand, usually 50% more. This kind of mould is used for making large parts like engine bodies, frames etc.
- Cemented-bonded moulds: In this kind if mould, the moulding sand contains 10-15% of cement which acts as a binder. These kind of moulds are used for creating large castings, but it is difficult to remove such moulds from the castings.
- CO2 moulds: In this moulding process, sodium silicate (Na2O.x SiO2) is used as a binder rather than clay. CO2 gas is made to flow through the mould after making the mould which then hardens the mould due to the following reaction.
The value of x in the above relation is 1.6 to 4, most often 2. The reaction takes place in just 1 minute. This is very quick in comparison to the several hours required to produce a dry sand mould. This kind of mould can be used to make intricate castings, as the sand mixture has a high flowability to fill up intricate contours on the casting body.
- Resin bonded sand mould: In this kind of moulds, the green sand mixtue is mixed with polymers or an oil like linseed or soyabean oil. The function of polymers or oils is that it bonds the sand particles together during the baking process. This makes the strength of polymerised resin greater than that of the pitch which is used in dry sand moulds. This means that the moulds produced are stronger. The purpose of baking is to obtain the required strength of moulds or cores, but many a times this can be achieved without baking. Such moulds are called Furan-no bake moulds.
- Dry sand core moulds: This kind of mould is used when the moulding flask is too big to fit in an oven for baking, the moulds are made from assemblies of sand cores. A sand core is prepared from core sand mixture and is baked at to for 4 to 24 hours.
In the next article, we will classify the different types of casting processes.