Sand casting and gravity die casting

A casting is the cheapest way to get a complicated shape in metal, and it is the process most often specified by people who have never watched one being made. Almost every difficult casting is difficult because of a decision taken in CAD, months before any sand was packed.

What is actually in the mould

Feature	What it does
Pattern	Forms the external cavity; withdrawn before pouring
Core	Forms internal passages the pattern cannot make
Sprue, runner, ingate	Carry metal into the cavity without turbulence
Riser / feeder	Holds liquid metal to feed solidification shrinkage
Chill	Cools a local region early, to steer the freezing order
Parting line	Where the two mould halves meet — and where flash appears

Every pattern is the wrong size on purpose

Metal contracts as it cools, so the cavity has to be made oversize — typically one to two per cent, depending on the alloy and how much the mould restrains it.

Two more allowances go in alongside it:

• Draft — one to three degrees of taper on vertical faces so the pattern can be withdrawn without dragging the sand face. Missing draft is the most common reason a model comes back from the foundry for revision.
• Machining allowance — a few millimetres of stock on faces that will be machined, so the cutter reaches clean metal below the cast skin.

Feeding — where castings actually fail

Metal shrinks when it freezes, not only when it cools. Unless liquid can flow in to replace that volume, a void forms in whatever solidifies last.

So the whole art is directional solidification: arranging matters so the freezing front moves steadily towards a feeder that stays liquid longest.

Sand or gravity die

	Sand casting	Gravity die casting
Mould	Sand, destroyed each pour	Metal, reused thousands of times
Tooling cost	Low	High
Cooling rate	Slow	Fast — finer grain, better properties
Surface finish	Rough	Good
Tolerance	Loosest	Better than sand
Economic at	Any volume, including one	Medium to high volume

A permanent metal mould pulls heat out far faster, which refines the structure and tightens the dimensions. It also has no compliance — sand crushes a little as the casting contracts, and a steel die does not — so contraction and ejection have to be designed rather than tolerated.

What tolerance a casting can hold

As-cast tolerances are quoted as grades that widen with size and differ by process. They are not machining tolerances and never will be.

The practical approach is the one that survives contact with a foundry:

• Let the as-cast dimensions carry the casting tolerance grade.
• Machine the features that need to be accurate, and give them the allowance to do it.
• Establish the machining datums on features the foundry can hold consistently — which is the datum target argument from datums and datum reference frames, and the reason datum targets exist on castings at all.

What to take away

• The pattern is oversize by the contraction allowance, and it belongs to one alloy.
• Cores make internal passages — and they must be located, vented and removable.
• Risers feed solidification shrinkage. They must be bigger and freeze later.
• Design for directional solidification, towards the feeder.
• Draft of 1 – 3° lets the pattern come out. Missing draft stops a quote.
• The parting line sets where draft is measured from and where flash lands.
• Leave machining allowance so the cutter reaches sound metal.
• Chills steer the freezing order; an isolated heavy section is a hot spot.
• Gravity die buys finish, grain and tolerance for tooling cost and volume.
• Expect a casting tolerance grade, not a machined tolerance.