Foundry
Foundry questions test solidification and feeding logic: can you design a mould that fills without turbulence, freezes in the right order, and avoids shrinkage and gas defects?
Core rules
- Chvorinov's rule: solidification time t = B(V/A)², where V/A is the modulus. A riser must freeze later than the casting it feeds, so its modulus must be larger, typically by a factor of 1.1 to 1.2 or more.
- Directional solidification means the mould and gating are arranged so freezing progresses towards the riser. Chills accelerate cooling locally to steepen the thermal gradient.
- Pattern allowances: shrinkage (added, positive), machining (added), draft (added for withdrawal) and distortion (added in the opposite sense).
- Sand properties: green strength, permeability, refractoriness, collapsibility. Cores must be permeable and collapsible to avoid hot tears on contraction.
- Typical defects: shrinkage porosity (feeding failure), gas porosity (moisture, low permeability), hot tears (restraint during contraction), cold shuts and misruns (low pouring temperature, poor gating), sand inclusions.
Worked example
A plate casting 20 × 10 × 5 cm has V = 1000 cm³ and A = 700 cm², so V/A = 1.43 cm. Required riser modulus = 1.2 × 1.43 = 1.71 cm. For a cylindrical riser with height equal to diameter, V/A = D/6. Hence D ≥ 6 × 1.71 = 10.3 cm. Specify a 105 mm diameter × 105 mm tall riser.
Traps that cost marks
Subtracting the shrinkage allowance instead of adding it. Sizing the riser from volume alone rather than modulus. Blaming gas porosity for what is actually shrinkage. Forgetting that a strong, non-collapsible core causes hot tears.