Moulding material may be defined as the material out of which mould is made.
It should be such that the mould cavity retains its shape until molten metal solidifies.
Casting can be done in
(i) Permanent mould – ferrous metal and alloys (steel, Gray CI).
(ii) Temporary mould – refractory sand and resin.
(iii) Wax, plaster of paris, carbon, ceramics, etc.
Permanent mould normally used for casting low melting point metals.
Permanent moulds are costly also.
Refractory moulding used for casting high melting point materials and bigger in size.
Most of the foundry output comes from temporary moulds.
Normally used refractory sands are
(i) Silica sand
(vii) Graphite / Carbon
Generally two types
Green sand mould
Dry sand mould
Green Sand Mould
– Green sand may be defined as sand which is in damp (wet) condition and contains moisture.
– Clay binder is added to it.
– In green sand mould, molten metal is poured while it is in wet/undried condition.
– A green sand mould has low strength and low permeability.
– Difficult shapes can be produced.
– Castings do not crack because there is less resistance to soild shrinkage.
– Suitable for small and medium size casting.
Dry Sand Mould
– It is shown in Fig.
– Binders are added which harden when heated.
– Mould is prepared in green (wet) condition and then heated in oven for drying (300° to 650°F) before pouring.
– It has more strength than green sand mould.
– More expensive.
– Mould gas generation is less than green sand mould.
– Smoother surface on casting because fine sand is used.
– Higher permeability than green mould.
– Preferred for large size casting.
Metal Moulding/ Dies
– These are also called permanent moulds.
– Generally made of grey cast iron/steel.
– Metal mould is made of two parts to facilitate removal of casting.
– Mould is manufactured by casting and then machining of the cavity.
– Generally used for casting nonferrous metals and alloys. (A1, Mg, Pb etc.)
– Ex : Aluminium alloy pistons (I.C. Engine) are cast in metal moulds.
– Metal moulds are used in the following type of castings.
(i) Permanent mould casting
(ii) Pressure die casting
(iii) Centrifugal casting
– Surface produced by metal mould have
(i) fine grain structure
(ii) high dimensional accuracy
(iii) very good surface finish
– Preferred for mass production of casting.
Process to be followed for investment moulding are as follows :
Use a master pattern to make metallic die cavity.
Pour molten wax in the die and get wax pattern.
Protecting of wax pattern is done by repeatedly dipping it in slurry of a fine refractory material.
Pour the investment moulding mixture around the protected wax pattern.
Allow investment to harden.
Invert the mould and heat it in the furnace (200° – 300°F).
It results in
(i) hardening of investment of mould
(ii) melting and removal of wax (can be reused)
Investment mould is heated from 1000°F to 1800°F.
Molten metal can be poured into it.
– In this process, mould and cores are prepared of thin shells using a mixture of fine sand and thermosetting resin, sand (100 – 150 mesh).
– This mixture is poured on a heated metal pattern.
– Some amount of mixture adhere to the pattern.
– The resin cures and gets bonded with each other and forms a shell like structure around the pattern.
– The portion of the shell touching the metal pattern gets the shape and size of the pattern and is one half of the mould.
– In the same manner the other half is also prepared.
– Two halves are placed together which constitute the mould assembly.
– The assembly is placed in a flask and backup material placed around it
– Now molten metal can be poured in.
– It is shown in Fig.
|Shell moulding process|