Mould making is a process of creating a replica of casting with the help of patterns and moulding sand. The cavity, produced in sand body, facilitates the molten metal to solidify and to take the shape of the cavity. Various types of moulding sands used in a foundry are classified as follows:
Green Sand: Green sand is a mixture of silica sand, clay, and water. Normally percentages of water and silica in green sand are 6% and 18%, respectively.
Dry Sand: Dry sand initially has high moisture content but the moisture has been evaporated from it by drying its mould in an oven.
Parting Sand: Parting sand is used on the parting plane to prevent the sticking of cope and drag part.
Baking Sand: This is already used sand in casting. Before reuse, it is riddled to remove all foreign materials and used to fill the moulding flask after facing sand has been rammed around the pattern.
Facing Sand: This is fresh prepared and tempered foundry sand and it is used all around the pattern and remainder may be green sand.
Types of Moulds
Mould is a cavity of heat resistant materials into which a molten metal is poured. Sand is most suitable for mould material due to heat receptivity, permeability, and low cost but metal moulds are also used for small non-ferrous and precision casting. The various types of moulds used in casting can be classified as follows:
Green Sand Mould: Green sand is a mixture of silica sand, clay, and water. The percentages of clay and water in the mixture vary from 10% to 12% and 3% to 6%, respectively. It is known as green sand due to wetness.
Skin-dried Moulds: In this mould, the cavity surface up to a depth of ½ inch is dried and hard. Generally, two methods are employed to prepare a skin dried sand mould. In the first method for preparing the skin-dried mould, the sand around the pattern to a depth of ½ inch is mixed with a binder so that when it is dried it will leave a hard surface on the mould. In the second method, the entire internal surface of the mould is coated by spray or wash with linseed oil, molasses water, gelatinized starch, etc., which harden on heating.
Dry Sand Moulds: Dry sand mould is made from coarse moulding sand with binding material. It is a mixture of green sand, cereal flour, and coal tar. The prepared mould is backed in an oven at 110–260°C for several hours for hardening. This type of mould is generally used for large steel castings. They give better surface finish and also reduce the incidence of the casting defects such as gas holes, blowholes, or porosity. However, due to the greater strength of these moulds, tearing may occur in hot-short materials.
Loam Sand Moulds: Loam sand moulds are used for large work like pit moulding. The mould is first built with bricks or iron parts. These parts are then plastered over with a thick loam mortar, the shape of the mould being obtained with sweeps or skeleton pattern. The mould is then allowed to dry thoroughly so that it can resist the wear due to heavy rush of molten metal.
Core Sand Moulds: Core sand moulds are made from core sand in subparts and assembled together. They are made in subparts due to difficulty in handling during baking.
Metal Moulds: Metal moulds are used in die casting of low melting temperature alloys. It may be used for ferrous and non-ferrous casting but it is more suitable for non-ferrous casting. Castings have smooth surface finish, accurate size, and better mechanical properties and are produced with faster rate of production. Thus, machining works are eliminated.
Special Moulds
CO2 Moulds: This is also a sand moulding in which water glass (Na2OxSiO2, sodium silicate) is used as binder. After mould is prepared, CO2 is made to flow through the mould and mould gets hardened. The chemical reaction for the process is
This is a one of the method of quick mould hardening.
Resin-bonded Sand Moulds: In these moulds, green sand mixture is mixed with thermosetting resins such as linseed oil. The resin is oxidized during baking and mould gets hardened due to polymerization.
Shell moulds: These moulds are prepared by heating a mixture of sand and resin over the surface of a metallic pattern. This enables the production of a thin and rigid layer of uniform thickness which, when separated from the pattern surface, forms one part of the shell. Two such parts are joined to form a complete shell mould.
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