Core is used in sand moulds in order to obtain the desired internal cavities or the shapes which can not be obtained otherwise like external projections or negative drafts. Core is also used in injection molding.
Types of cores:
Sand cores – They are generally made of green sand or dry sand. Green sand cores have relatively low strength. Dry sand core contains dry sand and binders (clay, organic or inorganic) so they develop strength on baking. The types of sand cores are
(i). Horizontal (ii). Vertical (iii). Balanced (iv). Drop
Metal cores – The metal core are generally made of cast iron or steel.
Sand core making:
The various steps involved in sand core making are
The cores are formed in core boxes similar to those used to make moulds.
Core baking – Cores are baked in ovens equipped with suitable holding devices. The temperature at which core is baked depends upon the core material used.
Core dressing – A compound is applied on the surface of core either in green state or after baking to protect it from molten metal and to provide a smooth surface in a cored hole.
Core chaplet – A metal location piece is inserted in a mould to provide extra support to core and prevent shifting from its position. The chaplet melts as it come in contact with the molten metal and forms part of the cast material.
The cores are supported in moulds by core prints. The core prints are provided as a projection at the end of core.
Properties of cores:
Permeability – Vents are provided in core to escape the gases generated during casting. The core should have more permeability than the mould itself.
Collapsibility – The core should collapse shortly after the molten metal has solidified. This is required for better contraction of the metal.
Thermal stability – The core should stand high temperatures of molten metals.
Dry strength – The strength of core after baking is known as its dry strength. The core should have sufficient dry strength when it came in contact with molten metal to retain the shape of cavity and to resist erosion.