Foundry and Casting: Patterns, Sand and Gating
Foundry and casting tests the sequence from pattern to finished casting, pattern allowances, moulding sand properties, and the purpose of the gating and risering system. Expect a calculation on shrinkage allowance and labelling of moulding tools.
Sequence
Pattern making, core making if needed, moulding, melting, pouring, solidification, shakeout, fettling (cleaning), inspection.
Pattern allowances
- Shrinkage (contraction) - added because liquid metal contracts on cooling. Typical values: grey cast iron 10 mm/m (1 %), steel 20 mm/m (2 %), aluminium about 16 mm/m.
- Draft - a taper of 1–3° on vertical faces so the pattern withdraws cleanly.
- Machining - extra metal where surfaces will be machined.
- Distortion - for U- or V-shaped castings that warp.
- Shake - a small negative allowance for rapping the pattern loose.
Moulding sand
Silica sand with clay binder and moisture. Essential properties: green strength, permeability (lets steam and gas escape), refractoriness, cohesiveness, adhesiveness and collapsibility. Too much moisture raises strength but lowers permeability and causes blowholes.
Gating and feeding
Pouring basin, sprue, runner and ingate feed the cavity. A riser holds a reservoir of liquid metal to feed shrinkage; a core forms an internal cavity and is supported by chaplets; vents let gas escape.
Worked example
A grey cast-iron casting is to be 500 mm long. Shrinkage allowance at 1 % is 5 mm, so the pattern length is 505 mm. If 3 mm is also left on each end for machining, the pattern becomes 505 + 6 = 511 mm. Add allowances to the pattern, never to the finished casting dimension.
Traps
Subtracting the shrinkage allowance. Confusing a riser with a sprue. Thinking draft is a shrinkage allowance. Forgetting that a core needs venting and chaplets. Adding machining allowance only on one face when both faces are machined.