Question one: how heavy is the part
Piece weight eliminates most of the options immediately. Shell moulding stops at about 50 kg. Lost foam stops at about 100 kg. Green sand covers 0.2 kg to 200 kg. Resin sand covers 100 kg to 3000 kg. The vacuum process reaches about 10 000 kg.
If your part weighs 800 kg, the conversation is already down to resin sand or vacuum process before anyone has looked at the geometry.
Question two: what tolerance does the drawing need
Green sand gives CT11 to CT14. Resin sand gives CT10 to CT11. Shell moulding, lost foam and the vacuum process all reach CT7 to CT9.
Be careful here. A drawing that calls for CT8 on every dimension will push the part into an expensive process when in reality only three features need it. Marking the critical dimensions and letting the rest sit at CT12 often saves more than any other decision in the project.
Question three: how complex is the geometry
Lost foam handles complex structures that would need many cores in a sand mould, because the foam pattern is consumed rather than withdrawn. That is its main advantage.
Investment casting handles fine detail and thin sections with a good as cast surface, which means less machining stock and less machining time. It is normally the right answer for complex parts below about 25 kg.
Question four: how many do you need a year
Tooling cost is spread over the batch. A pattern for resin sand is cheap compared with a die, so a low volume part goes to sand even when a die cast part would be cheaper per piece.
The crossover moves with part size. For a small aluminium part, a few thousand pieces a year can justify a die. For a 500 kg steel casting, sand moulding stays right at almost any volume.
Process selection table
What we do with your enquiry
We read the drawing against those four questions and come back with a proposed route, a tolerance class and a machining allowance in writing. Where the drawing forces an expensive process for no functional reason, we say so.
In some cases we also reduce cost on an existing part by moving it to a different process. If you have a part that has been produced the same way for years, it is worth asking.