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 cores5. 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.

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

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

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.