Investment Casting vs Sand Casting: When to Use Which
A cast iron pan comes out rough and a golf club head comes out smooth, though both were cast. The one rule that separates sand casting from investment (lost wax) casting, each process step by step, the shrinkage both have to feed, and four questions that pick the right one for your part.
Problem
A cast iron frying pan and a golf club head were made the same basic way. Liquid metal was poured into a mould, left to freeze, and the mould was broken away. Yet the pan comes out rough and grainy, and the club head comes out smooth, with thin walls and crisp edges, straight from the mould. The pan was sand cast and the club head was investment cast, and piece for piece investment is usually the more expensive of the two. Pick the wrong one for your drawing, and you either pay for detail nobody needed, or machine each face of a part that could have come out finished.
Quick Answer
The short version
Sand casting packs sand around a copy of the part, called a pattern, draws the pattern out and pours metal into the hollow, and the same pattern goes back in for the next mould. It is cheap to set up and takes castings from hand-sized to heavier than a car, but the sand leaves a rough surface and loose sizes, so the key faces get machined. Investment casting makes the pattern from wax, coats it in ceramic and melts the wax out; with nothing to pull free, the shell copies fine detail, thin walls and a smooth surface. The price is a new wax pattern and a new shell for each casting, days of coating, and a limit on size. So choose sand when the casting is big, the numbers are low, or you will machine it anyway, and investment when it is small and detailed and the shell costs less than the machining it saves.
Deep Dive
Both processes pour metal into a mould and break the mould to free the casting. What differs is how the mould gets made, and that one difference decides the rest. The map:
