Basic Design of Metal Processes
The pattern is a physical model of the casting used to make the mold.
China precision machining The mold is made by packing some readily formed aggregate material, such as molding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled with metal to become the casting.
If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities. As has been defined earlier, China precision machining a pattern is a replica of object to be made by the casting process, with some modification.
The main modifications are:
a) the addition of pattern allowances,
(b) the provision of core prints,
(c) the elimination of fine details which cannot be obtained by casting and hence are to be obtained by further process.
For all those casting where coring is required, provision should be made to support the core inside the mould cavity. One of the methods that is universally followed is to provide core prints where possible. In Fig 5.22 is shown an example of the provision of core prints. The size of the core prints to be provided is to be estimated based on the specific casting.
Fig.5.22 Typical job, its pattern and the mould cavity
A large variety of materials are used in foundries for manufacturing moulds and cores. They are: Moulding sand, System sand (backing sand), Rebounded sand, Facing sand, Parting sand, Core sand.
The properties of moulding sand
The choice of moulding materials is based on their processing properties. The properties that are generally required in moulding materials are:
Refractoriness: it is the ability of the moulding material to withstand the high temperatures of the molten metal so that it dose not cause fusion. Green strength: The moulding sand that contains moisture is termed as green sand.
The green sand should have enough strength so that the constructed retains its shape. Dry strength: When the moisture in the moulding sand is completely expelled, it is called dry sand. When molten metal is poured into a mould, the sand around the mould cavity is quickly converted into dry sand as the moisture in the sand immediately evaporates due to the heat in the molten metal. At this stage, it should retain the mould cavity and at the same time withstand the metallostatic forces. Hot strength: After all the moisture is eliminated, the
sand would reach a high temperature when the metal in the mould is still in the liquid state. The strength of the sand that is required to hold the shape of the mould cavity then is called hot strength. Permeability: During the solidification of a casting, large mounts of gases are to be expelled from the mould. The gases are those which have been absorbed by the metal in the furnace, air absorbed from the atmosphere and steam and other gases that are generated by the moulding and core sands. If the gases are not allowed to escape from the mould, they would be trapped inside the casting and cause defects. The moulding
sand should be sufficiently porous so that the gases are allowed to escape from the mould. This gas evolution capability of the moulding sand is termed as permeability.
Beside those specific properties, the moulding sand should also have collapsibility so that during the construction of the solidified casting, it dose not provide any resistance which may result in cracks in the casting, they should be reusable and should have good thermal China precision machiningconductivity so that heat from the casting is quickly transferred.