Casting pours or forces molten metal into a cavity and lets it solidify, and the processes differ mainly in what the cavity is made of and how the metal gets there. That single difference drives everything else: tooling cost, achievable tolerance, minimum wall thickness, surface finish and how many parts the route makes sense for.
The main processes
Sand casting uses a cavity formed in bonded sand, destroyed after each pour: cheap tooling, rough surface, loose tolerances, any size. Investment casting uses a ceramic shell around a wax pattern: excellent detail and tolerance, higher cost per part. Die casting forces metal into a steel die at pressure: thin walls, fast cycles, expensive tooling, low melting alloys. Permanent mould casting pours into a steel die under gravity, sitting between them.
Choosing between them
Volume decides first. Sand for ones and tens and for very large parts. Investment where geometry is complex and the surface matters, at moderate volume. Permanent mould for mid volume in aluminium. Die casting for high volume in aluminium, zinc or magnesium. Alloy narrows it too, since die casting is limited to metals that will not attack the die.
Design rules every casting shares
Uniform wall thickness, so the part solidifies evenly and does not tear or shrink into voids. Generous radii, because sharp internal corners are where hot tears start. Draft on every face so the pattern or the part can be withdrawn. And a shape that allows the metal to feed the last part to solidify, which is what risers exist to do and what a section that solidifies early will prevent.
Porosity and machining allowance
Every casting process leaves some porosity, from gas or from shrinkage, and where it sits depends on how the part solidified. That matters when a machined surface opens a void, when a part must be pressure tight, and when heat treatment would blister trapped gas. Leave sensible machining allowance and specify the integrity requirement rather than assuming the foundry knows how the part is used.
Questions people ask about types of casting
Which casting process should I choose?
Volume and size decide first: sand for low volume and large parts, investment for complex detailed parts, permanent mould for mid volume aluminium, die casting for high volume.
What is the most important casting design rule?
Uniform wall thickness with generous radii, so the part solidifies evenly. Most shrinkage voids and hot tears come from violating it.
Why do castings have porosity?
Gas entrained during filling and shrinkage as the metal solidifies. It matters where machining opens a void, where the part must seal, and where heat treatment would blister it.
How much machining allowance should a casting have?
More on sand castings than on die castings. Ask the foundry for their recommendation for the process and feature size rather than guessing.