Casting

Casting pours molten metal into a mould shaped like the part. Learn sand, die and investment casting, patterns and allowances, gating and risers, solidification time (Chvorinov's rule), and common defects.

Casting is the oldest metal-shaping process and still the cheapest way to make complex shapes: engine blocks, pump housings, turbine blades, manhole covers. Liquid metal fills a cavity, solidifies, and takes its shape.

Good castings depend on how metal flows in (gating), how it shrinks as it freezes (risers), and how the mould releases heat.

An analogy

Making ice cubes or jelly in a mould — but with metal at 700–1600 °C that shrinks as it freezes and can trap gas.

Casting processes

Expendable-mould processes break the mould each time; permanent-mould processes reuse a metal die.

• Sand casting: cheap, any size, rough finish (engine blocks, valve bodies).
• Investment (lost-wax): fine detail, good finish (turbine blades, jewellery, implants).
• Die casting: molten Al/Zn injected into steel dies at high pressure — high volume, thin walls (gearbox housings, phone frames).
• Centrifugal: spinning moulds for pipes and rings.

Patterns and allowances

Metal shrinks while cooling: steel ≈ 2 %, cast iron ≈ 1 %, aluminium ≈ 1.3 %. Patterns are oversized by shrinkage, plus machining allowance on surfaces to be machined, plus draft (1–3°) so the pattern can be withdrawn from sand.

Gating and risers

Sprue, runners and gates guide metal in smoothly to avoid turbulence and trapped oxide. Risers are reservoirs that solidify last and feed shrinking metal into the casting. Chvorinov's rule says thick, compact sections (high V/A) freeze slowly — so risers are made chunkier than the section they feed.

Defects and simulation

Shrinkage porosity, gas porosity, cold shuts, misruns, hot tears and inclusions are common defects. Directional solidification (chills, risers, gating layout) and casting simulation software (MAGMA, ProCAST) predict fill and freezing to eliminate them before cutting tooling. X-ray and CT inspect critical castings.

Formulas

• Chvorinov's rule: t_s = B · (V/A)²
• Riser rule of thumb: M_riser ≥ 1.2 × M_casting
• Sprue exit velocity: v = √(2 g h)
• Fill time: t = V_cast / (A_gate · v)
• Pattern size: L_pattern = L_part (1 + shrinkage) + machining allowance

Worked example: Solidification time of a cube

Given: Cube 100 mm; B = 2 min/cm²

Find: t_s and the riser modulus needed.

• V/A = 10³/(6 × 10²) = 1.667 cm
• t_s = 2 × 1.667² = 5.56 min
• Riser modulus ≥ 1.25 × 1.667 = 2.08 cm

Answer: ≈ 5.6 min; riser modulus ≥ 2.1 cm

Worked example: Fill time

Given: Casting volume 2 L; Sprue height 200 mm, gate area 3 cm²

Find: Metal velocity and fill time.

• v = √(2 × 9.81 × 0.2) = 1.98 m/s
• Q = 3 × 10⁻⁴ × 1.98 = 5.94 × 10⁻⁴ m³/s
• t = 0.002/5.94 × 10⁻⁴ = 3.4 s

Answer: ≈ 3.4 s

Common mistakes

• Wrong: Designing sudden thick-to-thin transitions. — Right: Keep wall thickness uniform with gradual transitions and fillets to avoid hot spots.
• Wrong: Forgetting draft on vertical faces. — Right: Add 1–3° draft so patterns and die-cast parts release.

Applications

• Automotive: Cast-iron and aluminium engine blocks, cylinder heads; 'giga-casting' of car underbodies.
• Aerospace & energy: Single-crystal investment-cast turbine blades.
• Infrastructure: Valves, pumps, pipe fittings, manhole covers.

Key takeaways

• Pattern = part + machining allowance, scaled by shrinkage, with draft.
• t_s = B(V/A)²; risers must freeze last.
• Smooth gating avoids turbulence and oxides.
• Uniform walls and fillets prevent defects.

Going further

• Vacuum & squeeze casting: Reduce porosity for structural, heat-treatable aluminium castings.
• Directional solidification: Controlled withdrawal from a furnace grows columnar or single-crystal turbine blades without grain boundaries.
• Giga-casting: Very large high-pressure die-cast body sections replace dozens of stamped parts.