Core binders: Core binders are the additives in the core sand which holds the sand grains together, provides the strength to the sand core, provides resistance to the erosion, breakage, and collapsibility of the sand core.
Important things about Foundry Core Binders
- Types of sand binders (or core binders)
There are two types of sand binders in casting.
- Organic binders
- Inorganic binders
- Organic binders are easily combustible by the heat of molten metal and they get destroyed too.
As the organic binders are combustible, they lead to collapsibility of the sand core.
- Inorganic binders are not combustible and they have good strength even at very high temperatures.
Inorganic binders also provide good resistance to erosion and so they do not lead to collapsing of the sand core.
- Inorganic binders include;
- Core oils
- Cereal binders
- In many core sand mixtures, the core oil such as linseed oil is used. The proportion of core oil is kept 0.5 % to 3 % by weight.
- The common mixture of sand cores contains 40 parts of river sand and 1 part of linseed oil.
- The core oil does not give much strength to its green state, but it definitely gives more strength to its backed state (which is known as baked strength).
- For good strength of the oil sand core it should be baked for around 2 hours at 170 °C to 190 °C.
- In order to provide the green strength to the sand cores, the cereal binders are used.
- While manufacturing the sand cores, the amount of cereal binder is always kept less than the amount of core oil.
- Sulphite binders are water soluble compounds of wood sugars which are produced as a by-product of paper making.
- Sulphite binders provide strength due to evaporation of moisture and thus produce hard surface hardness during baking.
- The proportion of these binders are kept around 1.5% in the sand. And they are used in green sand as well as dry sand.
- The pitch (which is a by-product obtained during the preparation of coke) is a core binder used as a ground powder for up to 3% or more. It provides strength to the core which can resist the high temperature of the molten metal.
- Plastic core binders include thermosetting plastics like urea and phenol formaldehyde are widely used in core making. These plastic core binders are mixed with other ingredients like silica flour, cereal binders, water and kerosene.
- The core binders, which are urea based, are baked at 160 °C to 170 °C, while the phenolic core binders are baked at 180 °C to 200 °C.
Urea based core binders and phenolic core binders, both are completely combustible due to heat of the metal.
- The core binders have properties like, moisture resistance, high adhesive strength, burn out characteristics and they also provide good surface finish to the core.
- Inorganic core binders includes;
- Silica flour
- Iron oxide
- Inorganic binders are used to achieve green strength, baked strength, hot strength and smooth surface finish.
- The commonly used simple type of core sand mixture is made up of moulding sand, 1% core oil, 1% cereal and 2.5% to 6% water.