Shell mould casting

Shell moulding is a modification of sand casting in which a relatively thin shell forms the mold cavity into which the molten metal is poured.Shell moulding, which is sometimes known as shell mold casting, is one of the most suitable casting processes for complex and other thin sections.

PROCESS:

Make a mixture of sand, resin, and catalyst. Sand consists SiO₂ and zircon. Resin consists phenol formaldehyde. The catalyst consists hexamethyl tetramine.
After mixing put it into dump box having the hot pattern(150°c or more) on top.

Flip the system horizontally. Sand is exposed to the hot pattern. Sand and catalyst melted and form a thermosetting plastic layer on sand particles.

After few sec flip system horizontal again. Shell was made

Separate the shell mold from the pattern using ejector pins

We get half-shell mold. Made another half by repeating all above steps.
Clamp the shell molds in the flask filled with filler as shown below.

Pour the molten metal into the shell mold. After solidified take it out from shell mold.

FUNCTION OF ADDITIVES:

The Additives used- coaldust, MnO₂, CaCO₃, Fe₂O₃
Improve casting surface finish
Resistance to thermal casting
Strengthen the mold

FUNCTION OF LUBRICANTS:

The lubricant used- calcium
Increase sand fluidity
Ease of removal of shell form pattern

SHELL THICKNESS:

Shell thickness is depended on the time of sand exposed to the temperature of the pattern. More the time, more will be the thickness of the shell.

Shell thickness is very less (up to 8mm max.). Hence permeability of mold is very high (i.e., gases could escape from it very easily). To avoid this small sand particle are used.

ADVANTAGES: