Tip-3: Shell should have good permeability
Permeability:-
This is a property of the shell as to how well it can vent, i.e., how well gases pass through the shell. And in other words, permeability is the property by which we can know the ability of the shell to transmit fluid/gases, like air and steam.
During De-Waxing:-
Good shell permeability allows steam to easily pass through it so that steam can quickly reach the wax surface and start to melt the wax. Good permeability also allows the molten wax to slightly pass through its layers. This reduces the wax pressure build-up, leading to lower incidents of shell cracking.
Other Advantages Of Using High Permeability Shells:-
Shell firing becomes more efficient as the hot gases can pass through the shell. During metal pouring the gasses escape easily through the shell. This helps very good detail reproduction, reduces problems of blowholes and airlock.
Conclusion:-
High permeability shells have considerable benefits over low permeability shells. It is generally good to build high permeability shells.
Tip-4: Shell Should Have Uniform Thickness
During de-waxing: Uniform shell thickness allows equal amount of steam to pass through it over the entire surface of the casting, so that wax melts uniformly and causes very efficient de-waxing.
Other advantages of shell with uniform thickness:-
- During shell firing the sintering of the shell is uniform. Problems related to Under-sintered and over-sintered areas in the shell are minimised.
- After metal pouring the cooling of the casting is uniform and predictable. This can contribute directly to improved casting yield.
- Knockout of the shell becomes easier.
- There is considerable saving in shelling material. In a non-uniform shell the thinnest section is good enough to support the casting. All the areas of extra thickness are actually unnecessary additional material. If this is the saving.
Conclusion:-
Shells of uniform wall thickness have considerable benefits over shells with nonuniform thickness. It is generally good to build highly uniform shells.
