Casting Processes & Defects

Casting is a high-yield GATE ME topic (~2 marks). Chvorinov's solidification rule, the gating/riser function, and defect identification are the reliably tested pieces.

Sand casting & solidification

Molten metal is poured into a mould cavity. Elements: pattern (makes the cavity, oversized for shrinkage/machining allowances), gating system (sprue, runner, gate — feeds metal), riser (feeder — supplies liquid to compensate solidification shrinkage), core (makes internal cavities).

Chvorinov's rule: solidification time t = k·(V/A)², where V/A = casting modulus (volume/surface area). A larger modulus → slower solidification, so the riser must have a larger modulus (solidify later) than the casting to feed it.

Shrinkage occurs in three stages: liquid, solidification, and solid contraction (pattern allowances cover it).

Special casting processes

• Die casting: metal forced into a metal mould under pressure — fast, precise, for non-ferrous.
• Investment (lost-wax): wax pattern in ceramic shell — excellent finish, complex shapes.
• Centrifugal: rotation forces metal outward — dense, for pipes/rings.

Common defects

Blowholes/porosity (trapped gas), shrinkage cavity (inadequate feeding), misrun (metal solidifies before filling), cold shut (two streams fail to fuse), hot tears, inclusions.

Exam Tricks & Tips

• 🎯 Chvorinov: t = k(V/A)² — solidification time depends on the square of the volume-to-surface (modulus) ratio.
• 🎯 Riser must solidify AFTER the casting — so give it a larger modulus (V/A); a directional-solidification design.
• 🎯 Riser feeds shrinkage; gating system feeds the cavity — don't confuse their roles.
• 🎯 Misrun = incomplete fill; cold shut = two streams not fusing — both from low pouring temperature/fluidity.
• 🎯 Investment casting = best finish/complex shapes; die casting = high volume, non-ferrous — match process to requirement.
• ❌ Common mistake: thinking the pattern is the same size as the casting — it is deliberately oversized for shrinkage and machining allowances (and may include draft/taper).

Expected exam pattern

A 1-mark Chvorinov solidification-time or defect-identification MCQ/NAT, and a 2-mark modulus/riser-design or gating problem. Pattern allowances and process-selection questions appear as MCQs.

Quick recap

Sand casting: pattern (oversized), gating (sprue/runner/gate feeds cavity), riser (feeds shrinkage), core. Chvorinov t = k(V/A)²; riser needs a larger modulus to solidify last. Special: die (pressure), investment (lost-wax, fine finish), centrifugal. Defects: blowholes, shrinkage, misrun, cold shut.

Cover the answer, recall, then check. 11 cards on casting for GATE ME.

Q1. State Chvorinov's rule.
A1. Solidification time t = k·(V/A)², where V/A is the casting modulus (volume/surface area) and k a mould constant.

Q2. Why must a riser have a larger modulus than the casting?
A2. So it solidifies last, remaining liquid long enough to feed the casting's shrinkage (directional solidification).

Q3. Function of the gating system?
A3. Sprue, runner, and gate deliver molten metal smoothly into the mould cavity, minimising turbulence and air entrainment.

Q4. Function of a riser (feeder)?
A4. To supply extra liquid metal that compensates for solidification shrinkage, preventing shrinkage cavities.

Q5. Why is the pattern larger than the final casting?
A5. To allow for shrinkage (contraction), machining allowance, and draft/taper for withdrawal.

Q6. What is investment (lost-wax) casting best for?
A6. Complex shapes with excellent surface finish and dimensional accuracy (e.g. turbine blades, jewellery).

Q7. What is die casting suited to?
A7. High-volume production of non-ferrous parts, forcing metal into a metal mould under pressure for speed and precision.

Q8. What is a misrun defect?
A8. The metal solidifies before completely filling the cavity — caused by low pouring temperature or poor fluidity.

Q9. What is a cold shut?
A9. A defect where two metal streams meet but fail to fuse, leaving a weak seam — from low temperature/fluidity.

Q10. What causes blowholes/porosity?
A10. Gas trapped in the casting (from the melt or the mould) that could not escape before solidification.

Q11. What does centrifugal casting achieve?
A11. Rotation forces metal against the mould wall, producing dense, defect-poor cylindrical parts (pipes, rings).

Casting pours molten metal into a mould to make complex shapes in one step — engine blocks, pump housings. GATE tests solidification (Chvorinov's rule), riser/gating design, and the common defects. The physics of freezing drives most of the numericals.

Core concept: a casting solidifies from the surface inward; controlling the freezing time and feeding shrinkage (via risers) determines whether the part is sound or defective.

Deep explanation

Beginner — the sand-casting system

Molten metal flows through the gating system (pouring basin → sprue → runner → gate) into the mould cavity; a riser (feeder) supplies extra metal to compensate shrinkage during solidification; cores form internal cavities. Patterns are made oversize to allow for shrinkage, machining and draft allowances.

Intermediate — solidification and Chvorinov's rule

Metal shrinks in three stages: liquid contraction, solidification shrinkage, and solid contraction. Chvorinov's rule gives solidification time:
t = B (V/A)ⁿ, with n ≈ 2,
where V/A is the volume-to-surface-area ratio (the "modulus") and B a mould constant. Consequence: a riser must solidify after the casting (larger V/A, i.e. bigger modulus) so it can feed shrinkage. Chills (heat sinks) locally speed freezing to promote directional solidification toward the riser.

Advanced — special processes and defects

• Special casting: die casting (high pressure, fast, thin metals), investment (lost-wax, precise), centrifugal (pipes), shell moulding, continuous casting.
• Common defects:
  • Shrinkage cavity: insufficient feeding (undersized riser).
  • Blowholes/porosity: trapped gas (poor venting, wet sand).
  • Cold shut/misrun: metal freezes before filling (low pouring temperature/fluidity).
  • Hot tears: cracking from restrained contraction.
  • Inclusions: slag/sand entrapment.
    Riser design ensures the riser modulus exceeds the casting modulus by ~1.2× so it feeds correctly.

Worked example

A cube-shaped casting of side 100 mm and a cylindrical riser must be sized so the riser freezes later. Compare solidification times using Chvorinov (n = 2). Casting: V = 0.1³ = 1×10⁻³ m³; A = 6×0.1² = 0.06 m². Modulus = V/A = 1×10⁻³/0.06 = 0.0167 m.
t_casting ∝ (0.0167)² = 2.78×10⁻⁴. For the riser to solidify later, its modulus must exceed 0.0167 m — e.g. a riser of modulus 0.020 m gives t ∝ (0.020)² = 4×10⁻⁴, i.e. ~1.44× longer, so it feeds the casting. This is why risers are made chunky (high V/A).

GATE relevance

Chvorinov's rule (t ∝ (V/A)²), riser-modulus design, gating, and defect identification (shrinkage vs blowhole vs cold shut) are core Manufacturing questions. The V/A modulus concept and its role in directional solidification are heavily tested.

Exam tricks & shortcuts

• t ∝ (V/A)² — the riser needs a larger modulus than the casting to freeze last.
• To increase solidification time, increase volume relative to surface area (chunkier shape).
• Match defect to cause: cold shut = low temperature/fluidity; blowhole = gas; shrinkage cavity = poor feeding.
• Mnemonic: "Big modulus freezes last — that's the riser's task."

Key formulas

• Chvorinov's rule: solidification time t = C(V/A)² (V = volume, A = surface area).
• Riser design: riser must solidify after the casting → (V/A)_riser > (V/A)_casting.
• Shrinkage allowance added to pattern dimensions.
• Gating (Bernoulli/continuity): A₁V₁ = A₂V₂; pouring velocity V = √(2gh).
• Common defects: shrinkage cavity, porosity, cold shut, misrun.

Chvorinov's rule sizes risers so they feed the casting; higher V/A ratio means slower solidification.

Worked Example

Problem: A cube-shaped casting of side 10 cm solidifies in a sand mould. Using Chvorinov's rule with a mould constant C = 2 min/cm², estimate the total solidification time. Briefly state the implication for riser design.

Solution:
Chvorinov's rule relates solidification time to the casting's volume-to-surface-area ratio (the "modulus"):
t = C·(V/A)ⁿ, with n = 2.

Compute the modulus of a cube of side a = 10 cm:
V = a³ = 10³ = 1000 cm³.
A = 6a² = 6 × 10² = 600 cm².
V/A = 1000/600 = 1.667 cm.

Solidification time:
t = C·(V/A)² = 2 × (1.667)² = 2 × 2.778 ≈ 5.56 min.

Riser implication: to feed shrinkage, the riser must freeze after the casting, so it needs a larger modulus (V/A). A common rule is to make the riser modulus about 1.2 times the casting modulus.

Answer: Solidification time ≈ 5.56 minutes; the riser must have a larger modulus so it solidifies last.