Casting

In short

Casting is the manufacture of a part by pouring or forcing molten metal into a cavity shaped like the part and letting it solidify. It is the shortest route from raw metal to a complex three-dimensional shape — engine blocks, pump casings, pipes, valve bodies, turbine blades — and often the only economic route for large or intricate parts. India is among the world's largest casting producers, with clusters at Coimbatore, Rajkot, Kolhapur, Belagavi, Howrah, Agra, Batala and Pune, so foundry and casting-design knowledge is directly employable. In exams it carries weight in Manufacturing Technology papers and in GATE (solidification time, gating and riser design, defects), and in interviews at Ashok Leyland, Tata Motors, Mahindra, Kirloskar, Nelcast, Brakes India, Craftsman Automation, BHEL and L&T.

Explained

Fundamentals, patterns and allowances

• Casting terms: *flask* (moulding box) made of cope (top), drag (bottom) and cheek (middle, for three-part moulds); *parting line*; *core* (makes internal cavities); *core print* (seat for the core); *chaplet* (metal support for a core, melts into the casting); *chill* (metal insert that locally speeds cooling); *vent* (lets gas escape).
• Pattern: replica of the part used to make the mould cavity; made of wood (pine, teak), metal (Al, CI), plastic, plaster, wax or polystyrene. Its overall size is usually bigger than the part, by the allowances below — bores are the exception (machining allowance is subtracted) and shake is negative.
• Pattern types: solid (single-piece), split (two halves on the parting line), match-plate, cope-and-drag, loose-piece (for overhangs), gated (several small castings on one pattern), sweep, skeleton (for very large shapes), follow-board, segmental.
• Allowances on a pattern:
  1. Shrinkage (patternmaker's) allowance — for *solid* contraction after freezing; added as mm/m (grey CI 7–10, steel 20–21, Al 12–13, brass 15–16, Mg ~13).
  2. Machining (finish) allowance — 1.5–6 mm per surface to be machined; added on external surfaces, subtracted from bores.
  3. Draft (taper) allowance — 1–3° (or 10–25 mm/m) on vertical faces so the pattern lifts without tearing the sand.
  4. Distortion (camber) allowance — pre-bending for U, V and long thin sections that warp.
  5. Shake (rapping) allowance — a *negative* allowance, because rapping the pattern enlarges the cavity slightly.
• Three shrinkages, distinguished: liquid shrinkage (cooling of liquid, fed by the riser), solidification shrinkage (phase change, fed by the riser — Al 6–7%, carbon steel 2.5–3%, grey CI from about −2% to slightly positive because graphite expands), and solid shrinkage (after freezing, compensated on the *pattern*).
• Core making: core sand (silica + binder: oil, resin, sodium silicate) baked or gas-hardened; cores must have strength, permeability, collapsibility and good surface. Core shift and flotation (buoyant force on a core = weight of displaced metal minus core weight) must be resisted by prints and chaplets.

Moulding sand, its properties and tests

• Green sand composition (typical): silica sand 80–90%, clay binder (bentonite) 5–10%, water 3–6%, plus additives (coal dust, dextrin, cereal binder, sea coal).
• Properties: *refractoriness* (resists metal heat), *permeability* (lets gas pass), *green strength*, *dry strength*, *hot strength*, *collapsibility*, *flowability*, *cohesiveness*, *adhesiveness* (sticks to the flask), *reusability*.
• Sand grades in the mould: *facing sand* (next to the pattern, finest), *backing/floor sand*, *parting sand* (stops cope and drag sticking), *core sand*.
• Tests (IS 1918 covers physical tests on foundry sands; check the current revision): sieve analysis → AFS grain fineness number; permeability number (2,000 cm³ air through a standard specimen); green compression strength; clay content (wash test); moisture content; mould hardness; hot strength.
• Binder/process variants: *green sand* (cheapest, most used), *dry sand* mould, *skin-dried*, *loam*, CO₂ (sodium silicate) process — 3–5% sodium silicate hardened by CO₂ gas to silica gel, no baking, poor collapsibility; *no-bake / furan / phenolic* self-setting sands; *cement-bonded* sand.
• Moulding machines: jolt, squeeze, jolt-squeeze, sand slinger (large moulds), high-pressure automatic lines, DISA vertical flaskless lines (common in Indian high-volume iron foundries).

Sand casting step by step, gating and risering

1. Make the pattern with allowances; make core boxes and cores.
2. Ram the drag around the pattern; roll over, apply parting sand, set the cope with sprue and riser pins; ram the cope.
3. Lift the cope, draw the pattern, cut the gating, set cores, vent, close and clamp (or weight the cope against metallostatic lift).
4. Melt and treat the metal; pour steadily and keep the basin full.
5. Cool, shake out, remove cores, cut off gates and risers (fettling), shot-blast, grind.
6. Heat-treat if required, inspect, machine.
7. Gating-system elements: pouring basin (with a dam or strainer core to hold back slag), tapered sprue, sprue well, runner, runner extension, skim bob, filter, ingates, risers, vents.
8. Why the sprue is tapered: the stream accelerates as it falls, so a straight sprue leaves the metal unable to fill the section; the low pressure created aspirates air and gas into the melt (aspiration effect). Taper keeps the sprue full — area ratio from Bernoulli and continuity.
9. Gating ratio = sprue-base area : total runner area : total ingate area. The choke is the smallest, flow-controlling section — and it is the area that the flow equation sizes.
   • Pressurised (e.g. 1 : 0.75 : 0.5, choke at the gates): back pressure keeps the system full, less air entrainment, smaller gates, but higher velocity and more turbulence/erosion. Common for grey iron.
   • Unpressurised (e.g. 1 : 2 : 4, choke at the sprue base): low velocity, gentle filling, needed for Al, Mg and steel; the system does not run full, so runners must sit in the drag and gates in the cope.
10. Riser (feeder) function: supply liquid metal to compensate liquid and solidification shrinkage; the riser must freeze later than the section it feeds. Types: *top* and *side*; *open* and blind (covered, often with a Williams core/whistler to break the vacuum); *live (hot)* riser — fed by the incoming stream, more effective — and *dead (cold)* riser.
11. Directional solidification is the controlling principle: freeze progressively *towards* the riser. Tools — risers on heavy sections, padding, chills (external or internal) on isolated heavy sections, insulating/exothermic riser sleeves, tapering of sections. Heuvers' inscribed-circle method checks that section sizes increase towards the feeder.
12. Fluidity (ability to fill thin sections) is measured by a spiral fluidity test; it rises with superheat and falls with wide freezing range, oxide films and low pouring temperature.

Solidification and the governing rules

• Nucleation and growth: solidification starts at mould walls (chill zone, fine grains), then columnar grains grow along the heat-flow direction, and an equiaxed zone may form at the centre. Grain refiners and inoculants (ferrosilicon for grey iron, Ti–B for aluminium) give finer, more uniform grains.
• Chvorinov's rule: solidification time ∝ (volume/surface area)². The ratio V/A is the modulus M; a thin plate freezes far faster than a sphere of the same volume.
• Riser design methods: (a) modulus method — M_riser ≥ 1.2 M_casting; (b) Caine's equation relating freezing ratio to riser-volume ratio; (c) NRL shape-factor charts; (d) feeding-capacity check — riser must hold enough liquid for the shrinkage, since only about 14% of a cylindrical riser's volume is usable.
• Dendritic growth and segregation: alloys freeze over a range, giving a mushy zone; interdendritic shrinkage shows as microporosity and composition differences as segregation (normal, inverse, gravity).
• Cast iron families produced in the foundry: grey (flake graphite, good damping and machinability), white (hard, Fe₃C), malleable, SG/ductile iron (Mg-treated, spheroidal graphite), compacted graphite, alloyed/Ni-hard. Grey iron to IS 210, SG iron to IS 1865, carbon steel castings to IS 1030, aluminium alloy castings to IS 617 (verify current revisions and grade designations).

Melting, pouring and metal treatment

• Cupola — the classic grey-iron melter: charge of coke, pig iron, scrap, ferroalloys and limestone flux; zones (bottom up) well, tuyere, combustion (oxidising), reducing, melting, preheating, stack. Coke-to-metal ratio about 1:8 to 1:10. Cheap and continuous, but limited composition control and high emissions.
• Induction furnace (coreless and channel) — now dominant in Indian iron and steel foundries: clean, close temperature and composition control, no combustion products.
• Electric arc furnace — steel castings; crucible/pit, tilting and rotary furnaces and reverberatory/holding furnaces — non-ferrous (Al, brass, bronze).
• Pouring temperature: liquidus plus a superheat that depends on alloy, section thickness and pouring distance — a few tens of °C for steel, over 200 °C for grey iron. Indicative pouring ranges: grey iron 1,350–1,430 °C; carbon steel 1,540–1,600 °C; aluminium alloys 680–750 °C; brass 1,000–1,100 °C.
• Metal treatment: degassing (Al — rotary degassing with argon/nitrogen, tablets), fluxing and slag removal, inoculation of grey iron with FeSi just before pouring, nodularisation of SG iron with Mg (sandwich, tundish-cover or cored-wire methods), modification of Al–Si with Sr/Na, grain refinement, ladle filters.
• Pouring practice: bottom-pour and teapot ladles keep slag back; keep the basin full; avoid interrupted pouring (causes cold shuts).

Special casting processes and where each wins

• Shell moulding: resin-coated sand dumped on a metal pattern heated to 200–260 °C; a 5–10 mm shell cures, is stripped and clamped. Excellent finish (3–6 µm Ra) and tolerance (±0.25 mm), thin sections, good for high volumes — crankshafts, camshafts, small iron parts.
• Investment (lost-wax) casting: wax pattern on a sprue tree → ceramic slurry and stucco coats → dewaxing → firing → pouring → shell knock-off. Very intricate shapes, tolerance to about ±0.1 mm, used for turbine blades, valve parts, surgical instruments, jewellery; expensive per piece.
• Plaster and ceramic-mould casting: fine detail for non-ferrous (plaster) and for steel/superalloys (ceramic).
• Full-mould / lost-foam: expanded polystyrene pattern in unbonded sand vaporises as metal enters; no parting line, no draft, no core assembly.
• Permanent (gravity) die casting: metal mould, gravity pour; Al pistons, cylinder heads, LPG valve bodies. Higher cost, better properties and repeatability than sand.
• Pressure die casting: metal injected into a steel die at high pressure and held.
  • Hot-chamber — pump submerged in the melt; Zn, Mg, Pb alloys; fast cycles.
  • Cold-chamber — melt ladled into a separate sleeve; Al, Cu alloys, which would attack a submerged pump; higher injection pressures.
  • Excellent finish and thin walls, very high die cost, gas porosity typical, so not usually weldable or heat-treatable; low-pressure die casting (0.2–1 bar from below) gives cleaner metal, used for alloy wheels.
• Squeeze casting and semi-solid (thixo/rheo) casting: low turbulence, near-zero porosity, forging-like properties.
• Centrifugal casting: (i) true centrifugal — rotating cylindrical mould, dense defect-free walls with impurities pushed to the bore; CI and DI water pipes (IS 1536 for spun iron pressure pipes, IS 8329 for ductile iron pipes — confirm current revisions), bushes, liners; (ii) semi-centrifugal — solid rotation-symmetric parts such as wheels and pulleys, centre is poorer so it is cored or machined; (iii) centrifuging — small parts arrayed around a central sprue.
• Continuous casting: steel from the tundish through a water-cooled oscillating mould into billets, blooms and slabs; used at Tata Steel, SAIL, JSW, JSPL. High yield, no ingot moulds.
• Slush casting (hollow decorative parts), vacuum casting, directional and single-crystal casting (turbine blades).

Defects, inspection, standards and safety

• Gas defects: blowholes (round internal cavities from mould gas — low permeability, high moisture, bad venting), pinholes (many small subsurface holes, usually hydrogen), air inclusion/aspiration, blister (blowhole breaking the skin).
• Shrinkage defects: shrinkage cavity (jagged, in the last-freezing heavy section — undersized or wrongly placed riser), centreline/micro porosity, sink on the surface.
• Filling defects: misrun (cavity not filled), cold shut/cold lap (two streams meet without fusing), cold shot (splashed globules), pour short.
• Mould and sand defects: swell (soft ramming), drop (sand falls into the cavity), scab and buckle (expansion defects), rat tail, metal penetration, fins/flash, mould shift / core shift / mismatch, crush, run-out.
• Thermal and metallurgical defects: hot tear/hot crack (tensile stress during freezing, sharp corners, hard mould), warpage, hard spots (chilled edges in grey iron), slag/dross/sand inclusions, shrinkage-induced residual stress.
• Standard remedies to quote: adequate venting and permeability, controlled moisture, tapered sprue and filters, pouring at the right temperature and rate, risers and chills for directional solidification, fillets at corners, uniform wall thickness, collapsible sand to avoid hot tears, clean dry charge.
• Inspection: visual and dimensional, leak/hydraulic pressure test (pipes, valve bodies), radiography (internal shrinkage, porosity — severity levels to standard reference radiographs), ultrasonic (wall thickness, internal flaws), magnetic particle (ferrous surface cracks), dye penetrant (non-magnetic), hardness and tensile test bars cast with the heat, spectrometer chemical analysis, metallography for graphite form and nodularity count.
• Design-for-casting rules: uniform sections, generous fillets, avoid hot spots at junctions (stagger ribs, use cores), place the parting line for easy drawing, allow draft, design for a simple core assembly, keep machined datums on one side of the parting line.
• Standards and regulation in India: BIS standards for cast materials and products (IS 210, IS 1865, IS 1030, IS 617, IS 1536, IS 8329 and others; grade designation systems are revised periodically, so always check the current edition). Foundry work is covered by the Occupational Safety, Health and Working Conditions Code, 2020 (OSH Code, in force 21 November 2025, which repealed the Factories Act, 1948) and the state Factories Rules saved under it — foundries are a listed hazardous-process industry and silicosis is a notifiable occupational disease, so dust control, exhaust ventilation and medical examination are statutory; cupola and induction-furnace emissions fall under the Air (Prevention and Control of Pollution) Act, 1981 and State Pollution Control Board consents. The Institute of Indian Foundrymen (IIF) is the industry body.

Key points

• Casting - pouring molten metal into a cavity and solidifying it to shape; the cheapest route to complex geometry.
• Pattern ≠ part - it carries shrinkage, machining, draft, distortion and (negative) shake allowances.
• Patternmaker's shrinkage - compensates *solid* contraction only; liquid and solidification shrinkage are fed by the riser.
• Green sand - silica + bentonite + water; must balance permeability against strength and finish.
• Permeability number - standard AFS measure of how freely gas escapes the mould; too low gives blowholes.
• Core - makes hollow features; needs collapsibility and support against buoyancy (chaplets).
• Tapered sprue - prevents aspiration of air that a free-falling stream would suck in.
• Gating ratio - sprue base : runner : ingate; pressurised (1:0.75:0.5, choke at the gates) for iron, unpressurised (1:2:4, choke at the sprue base) for Al and steel.
• Riser - must solidify *after* the section it feeds and hold enough metal for the shrinkage.
• Chvorinov's rule - t = B(V/A)ⁿ, n ≈ 2; modulus V/A governs freezing time, not volume alone.
• Modulus rule of thumb - M_riser ≥ 1.2 M_casting, i.e. about 44% longer freezing time.
• Directional solidification - freeze towards the riser, using chills, padding and sleeves.
• Chill - speeds local freezing to kill an isolated hot spot; in grey iron it can cause hard spots.
• Shell moulding - resin-coated sand on a hot metal pattern; fine finish, high volume.
• Investment casting - lost wax, ceramic shell; intricate, close tolerance, costly.
• Die casting - hot chamber for Zn/Mg, cold chamber for Al/Cu; fast and accurate but gas-porous.
• Centrifugal casting - rotation gives dense pipe walls with inclusions at the bore.
• Blowhole vs shrinkage cavity - smooth and round (gas) versus jagged in the last-freezing section (feeding).

Formulas and examples

Example 1 — solidification time and riser size (plate casting). Grey iron plate 200 × 100 × 20 mm, mould constant B = 2.0 s/mm².

• V = 200 × 100 × 20 = 400,000 mm³. A = 2(200×100) + 2(200×20) + 2(100×20) = 40,000 + 8,000 + 4,000 = 52,000 mm².
• M_c = V/A = 400,000/52,000 = 7.69 mm → t_c = 2.0 × 7.69² ≈ 118 s (≈2 min).
• Riser by the time criterion (t_r ≥ 1.25 t_c): M_r ≥ √1.25 × 7.69 = 8.60 mm; with V/A = D/6 → D = 6 × 8.60 ≈ 52 mm, H = 52 mm.
• Cross-check by the modulus rule (1.2 M_c = 9.23 mm): D ≈ 55 mm — so a 55 mm × 55 mm riser is the safe choice.
• Feeding check for the same casting in steel (3% solidification shrinkage): metal needed = 0.03 × 400 cm³ = 12 cm³. Riser volume = πD³/4 = π(5.5)³/4 ≈ 131 cm³; usable at ~14% efficiency ≈ 18 cm³ > 12 cm³ — adequate.

Example 2 — sprue and gating design. Casting 1,200 cm³ of grey iron (≈8.6 kg at 7.2 g/cm³), pouring time 10 s, basin depth 20 mm, sprue height 180 mm (so h_base = 200 mm, h_top = 20 mm).

• v at the choke = √(2 × 9.81 × 0.20) = √3.924 = 1.98 m/s.
• Flow rate Q = 1,200/10 = 120 cm³/s = 1.2 × 10⁻⁴ m³/s.
• Ideal choke area = Q/v = 1.2 × 10⁻⁴ / 1.98 = 6.06 × 10⁻⁵ m² = 60.6 mm²; with c = 0.8, A_choke = 60.6/0.8 = 75.7 mm² → d ≈ 9.8 mm (use 10 mm).
• A pressurised 1 : 0.75 : 0.5 system chokes at the gates, so total ingates ≈ 76 mm² (two gates of 38 mm²), runner = 0.75 × 152 = 114 mm², sprue base = 152 mm² → d_base ≈ 13.9 mm.
• Sprue top area = 152 × √(200/20) = 152 × 3.162 = 481 mm² → d_top ≈ 24.7 mm.

Example 3 — pattern allowances (grey iron pulley). Finished part: OD 300 mm, length 400 mm, bore 60 mm. Shrinkage 10 mm/m (1.0%); machining allowance 3 mm per surface on OD, both ends and the bore.

• OD: 300 × 1.01 = 303.0, add 2 × 3 → 309.0 mm.
• Length: 400 × 1.01 = 404.0, add 2 × 3 → 410.0 mm.
• Bore: 60 × 1.01 = 60.6, subtract 2 × 3 → 54.6 mm (so machining removes metal, not air).
• Plus 1–3° draft on vertical faces and a small negative shake allowance.

Example 4 — permeability number. 2,000 cm³ of air passes a standard specimen in 30 s at a pressure of 10 g/cm².

• P = (2,000 × 5.08)/(10 × 20.268 × 0.5) = 10,160/101.34 ≈ 100 — typical of green sand for iron castings (roughly 50–150 depending on section thickness).

Example 5 — centrifugal casting speed. A CI pipe is cast in a mould of 200 mm internal diameter; required G-factor 60.

• N = 42.3 √(60/0.2) = 42.3 × √300 = 42.3 × 17.32 ≈ 733 rpm.
• Check: ω = 2π × 733/60 = 76.8 rad/s; G = ω²R/g = (76.8² × 0.1)/9.81 = 589.8/9.81 ≈ 60 ✓.

Where it is used

• Automotive and tractor foundries: cylinder blocks and heads, brake drums and discs, hubs, housings, exhaust manifolds — Ashok Leyland, Tata Motors, Mahindra, Nelcast, Brakes India, Rane, Craftsman Automation, Sundaram Clayton (die casting), Endurance and Sansera.
• Water and sewerage infrastructure: centrifugally cast DI and CI pressure pipes and fittings (Electrosteel, Jindal Saw, Tata Metaliks), valve and pump bodies for Jal Jeevan Mission-type projects.
• Pumps, compressors and valves: Kirloskar Brothers, KSB, Shakti and Coimbatore's pump cluster — grey iron and bronze casings cast in green sand and shell moulds.
• Power and heavy engineering: BHEL and L&T cast turbine casings, valve bodies and large steel castings; investment-cast blades and vanes for aero and gas turbines (HAL and its vendors).
• Steel plants: continuous casting of billets, blooms and slabs at Tata Steel, SAIL, JSW, JSPL — plus ingot and roll casting.
• Job roles: foundry/methods engineer (gating and riser design, simulation in MAGMA, ProCAST, AutoCAST), pattern and tool engineer, melting and metallurgical control, quality/NDT engineer, die-casting process engineer, and sand-plant and reclamation engineer.

Interview questions

• Q: Why is the sprue tapered and not straight? A: The falling stream accelerates and narrows; a straight sprue would then be only partly full, and the resulting low pressure sucks air and mould gas into the metal (aspiration). A taper matching √h keeps the sprue running full.
• Q: Which freezes faster — a 1-litre sphere or a 1-litre plate 25 mm thick? A: The plate. Chvorinov's rule makes time proportional to (V/A)²; the plate's modulus is about 10 mm against the sphere's 20.7 mm, so it freezes roughly four times faster.
• Q: How do you decide riser size? A: Make it freeze later than the section it feeds (M_r ≥ 1.2 M_c, or at least t_r ≥ 1.25 t_c) and check it holds enough liquid for the shrinkage, remembering only about 14% of a cylindrical riser's volume actually feeds.
• Q: Difference between a blowhole and a shrinkage cavity? A: A blowhole is a smooth, rounded gas cavity caused by mould gas or dissolved gas; a shrinkage cavity is jagged, sits in the last-freezing heavy section and means the feeding was inadequate.
• Q: Why does grey iron need less feeding than steel? A: Graphite precipitating during the eutectic freeze expands and offsets much of the solidification contraction, so grey iron's net shrinkage is near zero while carbon steel loses 2.5–3% by volume.
• Q: Hot-chamber versus cold-chamber die casting? A: In hot chamber the injection system sits in the melt, suiting low-melting zinc and magnesium; aluminium and copper alloys would attack it, so a cold chamber with a separate ladled sleeve and higher pressure is used.
• Q: Why are die castings usually not heat-treated or welded? A: Entrapped gas from very fast injection leaves fine subsurface porosity, which expands and blisters at high temperature.
• Q: What does a chill do, and what is its risk in grey iron? A: It locally speeds solidification to remove an isolated hot spot; too strong a chill makes white iron, giving hard spots that break tools in machining.
• Q: Why is machining allowance added on an outside diameter but subtracted from a bore? A: Machining removes metal from both; extra metal outside means a larger pattern OD, while extra metal inside means a smaller pattern core/bore.
• Q: Pressurised versus unpressurised gating — which for aluminium? A: Unpressurised, roughly 1:2:4, because aluminium forms tenacious oxide films and must fill slowly with minimum turbulence; pressurised systems are usual for grey iron.
• Q: Why do centrifugally cast pipes have clean outer walls? A: Centrifugal force throws the denser metal outwards and drives lighter slag, oxides and gas to the bore, which is then machined or allowed as the inner surface.
• Q: Name a statutory concern unique to foundry work in India. A: Silica dust exposure — foundry work is a listed hazardous process under the OSH Code, 2020 (which replaced the Factories Act, 1948 from 21 November 2025), with silicosis a notifiable occupational disease, so exhaust ventilation and periodic medical examination are mandatory; furnace emissions also need State Pollution Control Board consent.

Common mistakes

• Treating the pattern as the same size as the part, or adding shrinkage twice (once as "shrinkage" and again as "solid contraction").
• Using volume alone instead of V/A in Chvorinov's rule, or forgetting that surfaces not losing heat (a riser's contact face, an insulated top) must be excluded from A.
• Measuring the sprue head from the top of the sprue rather than from the liquid level in the basin, and mixing mm with m in √(2gh).
• Confusing the choke location: in a pressurised system the gates are the choke, in an unpressurised system the sprue base is — the flow equation sizes whichever one controls.
• Calling every internal hole a blowhole — the shape and location distinguish gas porosity from a shrinkage cavity, and the remedies (venting versus risering) are completely different.
• Oversizing risers "to be safe", which destroys yield; and placing a riser on a thin section, where it cannot feed the adjacent heavy section.
• Quoting die-casting tolerances or finishes for sand castings in an interview, or claiming die casting suits low volumes despite the die cost running into lakhs of rupees.

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