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
Horizontal (ii). Vertical (iii). Balanced (iv). Drop
Metal cores – The metal core are generally made of cast
iron or steel.
Sand core making:
various steps involved in sand core making are
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.
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.