when we have to go for casting and when to moulding?

It's the same as any other choice... work out which is capable of doing the job first.
If both are capable then compare tooling cost, set up time, delivery, price, quality, etc and go for the most effective.

Generally casting would be for metals and moulding for plastics, but with modern plasics they are interchangeable for some components, there may well be other alternatives, e.g Pressed steel.

Recently I redesigned a troublesome moulded plastic part as a pressed steel alternative. The resulting part had the following attributes...
Performance, better
Tooling cost, better
Design time/Tooling design/manufacture better
Piece part cost, worse.

So you can see it is always a trade off...our driving concern was getting a quick solution to a quality problem.

The problem doesn't go away even if you know what process... as you may still have to decide between factory A and factory B..

Apply the same technique again...

I like factory B best, although A is admirable

Del

in terms of plastics - examples might help to clarify your direction beyond simply cost -

Casting might be the better choice for items that might be:

a) lower in volume

b) require less 'precision'

c) extremely clear

d) made from 'thermoset' plastics; these are 'one time only' conversion to solids, can't be reformed again later

- an example would be clear acrylic and catalyst for paper weights or urethanes requiring a chemical reaction to solidify, the resin and catalyst for 'fiberglass' type applications etc.

molding would probably be the better choice for items that re

a) higher in volume

b) require more 'precision'

c) could be clear at substantial tooling pricing increase or required multiple surface finishes without additional machining

d) made from 'thermoplastic' plastics; materials that can be softened and reprocessed by reheating more than one time

- an example would be polystyrene CD cases

This is not to say that thermosets and fiberglass can't be molded - they can, again with advantages to the outcomes, but at considerably higher costs.

Del's answer on casting in terms of metals (aluminum, brass, bronze, pewter etc) and costing in general is correct - for instance the sand casting process can make some pretty intricate shapes requiring only minimal additional machining.

Jim

When you refer to 'moulding' is it 'forging' that you are actually referring to?

Some metals are difficult to cast.

Forging gives some preferred molecular structure as well as prestressing if required.

Forging can minimise machining.

Forging can keep better tolerances than casting.

Surface can be improved much by forging.

Forging can be done cold as well.

Forging can allow for heat treatment as well.

Castings may be cheaper.

Castings may be less prone to stresses in the final product.

Castings can have cavities, hollows and voids which forging could not achieve easily.

Alluminium, cast iron, steel, bronze, brass and lead alloys cast easily enough for most applications.

The metal has the first say in making the choice and the final product has the last say in determining which process is better.