IIIE SECTION A MANUFACTURING TECHNOLOGY NOTES

1. Core Moulding
A core is a predetermined shaped mass of dry sand which is made separately
from mould. It is positioned in a mould to obtain a shape in the casting which
can’t be readiliy obtained by the proper mould.
It requires core box pattern equipment, baking equipment and handling facilities.
Cores should be of sufficient thickness and free of fragile or overhanging
projections which might be easily broken during the necessary handling and
transportation involved in the production.
A core is used for the follwing purposes :
1. To reduce metal erosions in gates and runners. ( Splash Cores)
2. To retard foreign matter in the melt.(Strainer cores) ,
3. Provide a cup or the basin on top of the mould( Pouring basin cores)
4. In pit moulding the entire mould is made up of cores
5. To obtain the desired cavities and recesses which otherwise could not be
achieved by regular moulding operation
Materials used for Cores :
The main materials used for a core is refractory, either a natural or
synthetic sand.
The sand mix used for cores consists of sand mixed with binder
which gives strength to the core after baking.
A natural binder is linseed oil (sometimes called as Core-gum)
Synthetic binder is synthetic resin of the thermosetting type.
Sand is mixed with sodium silicate solution so that the grains are
coated and obtain a necessary hardness by “Gassing” with carbon
dioxide, causing the sodium silicate to bond the grains together and
produce strong core ,thus eliminating the baking process.
Binder is an agent added to the sand to produce a good green bond
and also hardness after baking.

2. Types of Cores :Cores are generally made up of green samd or dry
sands.The green sand cores have relatively low strength having
made of green sand , where as dry sand cores are made of sand and
binders, which develop strength when baked. These can also be
classified according to their position or use, according to which
these can be either horizontal, vertical, balanced,drop and other
types.
Core making : Cores are made separately in a core box, made of
wood or metal, The various steps in the core making are
Ramming of core sand in the core box,
venting,reinforcing,removing of core from
box,baking,pasting,sizing etc.
This work of producingcores can either done by hand or by some
machines designed for this purpose.in machine moulding the core
sand mixture is rammed by jolting,squezzing,or blowing by means
of suitable machine.venting, reinforcing and other operations are
carried by hand, cores of symmetrical cross section can be
produced by extruding core sand mixture through a suitable die
opening.the cores by extrusion process are made in lengths and cut
to shorter lengths as needed.
Cores are generally madeby core blowing machine for production
work, in which the core sand is placed in small hooper, over the
clamped core box, and blown under a pressure 5 – 8 kgf/cm2.The
core boxes are of special construction to provide for eascapment of
air. It requires special drying equipment.Even complex cores can
be produced in a few seconds by core blowing methd.
Core boxes used for core baking are designed in several types
ahich aid in core removal.The most commonly used are of split,
rollover and hooked type

3. Core Baking : Generally baking is carried out in ovens
equipped with drawers , shelves or other holding devices.The
operation is generally continuous and cores are put either in
batches or continuously over moving shelves. The heat in oven is
produced by burning oil or coke or by electrical resistance. Core
baking time depends on the types and quantity of binder used, the
amount of moisture used in sand and size of core. The temp for
baking depends on the core material used.When cores are baked,
they are more easily supported on a flat surface which should be
incorporated in the design.
Core dressing :Core dressing is the operation of applying a
compound to the surface of a core, either in the green state or after
baking for the purpose of providing protection against the scouring
action of flowing molten metal and to assist the formation of a
smooth surface in a cored hole or cored form
Core Chaplet : A Core chaplet is a metal location piece,
inserted into a mould either to prevent a core shifting its position or
to give extra support to a core. The molten metal melts the chaplet
which then forms part of the cast material.
Method of supporting cores in the mould :Generally
core prints are provided in cores. These core prnts are a sort of
projections at the end of a core and these support the core in
position in the mould. Design should provide space for core prints
sufficiently large to locate the core accurately, prevent sagging and
resist the hydrostatic molten – metal pressure.In some cores small
metal props ( Chaplets) are also used which are placed in a mould
cavity to support the cores and these props fuse and become part of
the casting.The chaplets must be therefore clean, dry and selected
of proper material. Sometimes small metal insert hinge tubes are
also used to provide support for tiny hole forming cores. As far as

4. possible chaplets should be avoided as fusing is not always easily
accomplished and it may lead to production of unsound casting.
Some cores are nearly fully surrounded by metals and a vent then
become necessary to release the gasses generated during pouring.
If possible the hole is formed by the vent core should be
incorporated as functional part of the design or located where it
will be do no harm, otherwise welding or plugging is required.
Charecterstics of cores : A core should have the follwinng
charecterstics:
1. Permeability : A core is supposed to be more permeable than
the mould itself and for this reason, coarse sand with large grain
size,( Spl washed silicon sand free from clay) mixed with molasses
is used for core making,Vents are also provided for giving easy
eascape of hot gasses that are generated during casting.
2. Collapsibility : A core should be capable of collapsing shortly
after the molten metal has solidified around the core. Collapsibility
provided freeness in the contraction of the metal.
3. Thermal stability : According to the service conditions to
which a core is subjected , core sand should be highly
refractory,otherwise defects like rough surface and metal
penetration etc may occur.
4. Core must have high dry strength or be capable of being handled
after drying. If required wires must be inserted into the core while
it is being moulded to give added strength.