Special casting: die, investment, centrifugal, shell

Why it matters

Sand casting is versatile but rough and slow. Car transmission housings, turbine blades, cast-iron pipes and crankshaft blanks are made by special processes chosen for accuracy, surface finish, mechanical properties or production rate. Choosing the right process — and knowing its limits on metal, size and quantity — is a routine manufacturing-engineering decision and a regular GATE question.

Key ideas

Expendable mould vs permanent mould. Sand, shell and investment moulds are destroyed to remove each casting (expendable mould). Gravity die (permanent mould), pressure die and centrifugal moulds are reused thousands of times, so they need metal or graphite dies and suit large quantities.

Shell moulding. Fine silica sand coated with a thermosetting phenolic resin is dumped onto a metal pattern heated to about 200–250 °C. A shell 5–10 mm thick cures on the pattern, the excess sand is dumped back, the shell is cured further and stripped. Two shell halves are glued or clamped and backed with shot or sand for pouring. Results: better accuracy (about ±0.25 mm on small parts) and finish than green sand, good permeability, high production with automation. Limits: metal patterns are costly, resin is expensive, castings are small to medium. Used for crankshafts, camshafts, valve bodies.

Investment (lost-wax) casting. Wax patterns are injected in a metal die and assembled on a wax sprue into a "tree". The tree is dipped repeatedly in ceramic slurry and stuccoed with refractory grains to build a shell (or invested in plaster for jewellery). The wax is melted out (dewaxing, usually in an autoclave), the shell is fired to burn out residue and preheated, then metal is poured into the hot shell. After cooling the shell is broken off. Advantages: very complex shapes, no parting line, excellent finish and accuracy, any alloy including high-melting superalloys, thin walls. Limits: expensive per piece, size usually small to medium. Used for turbine blades, surgical implants, jewellery and pump impellers.

Gravity die (permanent-mould) casting. Metal is poured under gravity into a reusable metal mould (cast iron or die steel), preheated and coated. Faster cooling gives finer grains and better properties than sand. Used for aluminium pistons and similar non-ferrous parts. Cores can be metal (simple) or sand (semi-permanent mould).

Pressure die casting. Molten metal is injected into a hardened steel die at high pressure (roughly 10–150 MPa) and held under pressure while it freezes. Very high rates, thin walls (down to about 0.5–1 mm), excellent accuracy and finish, little or no machining.

• Hot-chamber machine: the injection cylinder (gooseneck) sits immersed in the molten metal. Very fast cycles. Only for low-melting metals that do not attack the steel plunger — zinc, tin, lead and magnesium alloys.
• Cold-chamber machine: a measured shot is ladled into a separate shot sleeve and rammed into the die. Used for aluminium, brass and copper alloys, whose melts would attack an immersed pump or are too hot.
• Limits: very high die and machine cost (economic only for large quantities), porosity from trapped air (so parts are usually not heat-treated or welded), and ferrous metals are generally not die-cast because they would destroy dies.
• The die must be clamped with a force at least equal to injection pressure × projected area of the cavity and runners on the parting plane.

Centrifugal casting. Metal is poured into a rotating mould.

• True centrifugal — mould spins about its own axis (usually horizontal); no core is needed, the bore forms by centrifugal force, and its inner surface is a true cylinder. Pipes, liners, bushes. The outer surface is dense and fine-grained; light inclusions and slag move to the bore and are machined off.
• Semi-centrifugal — axisymmetric parts such as wheels and pulleys spun about their axis, with a central core if needed; the densest metal is at the rim.
• Centrifuging — several small mould cavities arranged around a central sprue and spun, to force metal into intricate cavities; not necessarily symmetric. Speed is chosen by the G-factor (ratio of centrifugal to gravitational acceleration), typically about 60–80 for horizontal true centrifugal casting. Too low a speed lets metal "rain" from the top; too high causes longitudinal cracks.

Other processes briefly: slush casting (pour out the unfrozen centre to get hollow ornaments), squeeze casting (pressure applied during solidification, near-forged properties), full-mould / lost-foam (polystyrene pattern vaporised by the metal), continuous casting (billets, slabs).

Formulas

G = ω²·R / g — G-factor (dimensionless); ω = angular speed (rad/s), R = inside radius of the mould (m), g = 9.81 m/s².

N = (60 / 2π) × √(G·g / R) — mould speed in rpm for a chosen G-factor.

F = m·ω²·r — centrifugal force on mass m (kg) at radius r (m), in N.

p = ρ·ω²·(r_o² − r_i²) / 2 — pressure exerted by the rotating liquid layer on the mould wall (Pa); ρ = liquid metal density (kg/m³), r_o = outer (mould) radius, r_i = inner (bore) radius (m).

F_clamp ≥ p_inj × A_projected — die-casting clamping force (N); p_inj = metal pressure in the die (Pa), A_projected = projected area of cavity plus runners on the parting plane (m²). A safety margin of 10–20% is usual.

Worked examples

Example 1 (standard — clamping force). An aluminium housing has a projected area of 200 × 100 mm on the parting plane (including runners). The cavity pressure is 30 MPa. Find the minimum clamping force, and the force with a 20% margin.

1. A = 0.2 × 0.1 = 0.02 m².
2. F = p × A = 30 × 10⁶ × 0.02 = 6.0 × 10⁵ N = 600 kN.
3. With 20% margin: 1.2 × 600 = 720 kN.

Answer: 600 kN minimum; about 720 kN machine rating.

Example 2 (GATE level — mould speed). A cast-iron pipe is made by horizontal true centrifugal casting in a mould of inside diameter 400 mm. A G-factor of 65 is required. Find the speed in rpm.

1. R = 0.2 m.
2. ω = √(G·g / R) = √(65 × 9.81 / 0.2) = √3188 = 56.46 rad/s.
3. N = 60 × 56.46 / (2π) = 539 rpm.

Answer: N ≈ 539 rpm.

Example 3 (wall pressure). Liquid steel (ρ = 7000 kg/m³) forms a layer from r_i = 180 mm to r_o = 200 mm in a mould spinning at 600 rpm. Find the pressure on the mould wall.

1. ω = 600 × 2π/60 = 62.83 rad/s.
2. r_o² − r_i² = 0.04 − 0.0324 = 0.0076 m².
3. p = 7000 × 62.83² × 0.0076 / 2 = 1.05 × 10⁵ Pa.

Answer: p ≈ 105 kPa — about one atmosphere, enough to feed the casting and push light inclusions toward the bore.

Common mistakes

• Putting aluminium in a hot-chamber machine. Molten aluminium attacks the immersed steel injection parts; aluminium is cold-chamber.
• Using the outside diameter of the pipe instead of the mould's inner radius in the G-factor.
• Forgetting to convert rpm to rad/s (×2π/60).
• Using the total surface area of the part, instead of the projected area on the parting plane, for clamping force.
• Assuming die castings can be heat-treated like sand castings — entrapped gas porosity blisters on heating.
• Saying investment casting has a parting line. The one-piece ceramic shell has none.

For GATE PI

Expect one-mark matching questions (process → product: turbine blade → investment, pipe → true centrifugal, carburettor body → die casting, crankshaft → shell), hot vs cold chamber selection by metal, and the role of wax, resin or rotation. Numericals ask for mould speed from a G-factor, centrifugal force or pressure, and die-casting clamping force. Practise rpm–rad/s conversions and the projected-area idea.

Quick check

1. Which metals are cast in hot-chamber die-casting machines?
2. In true centrifugal casting, where do light inclusions collect?
3. Mould inner radius 0.15 m, G = 70: speed in rpm?
4. Why does investment casting suit superalloy turbine blades?
5. What is the shell made of in shell moulding?

Answers: 1. Low-melting alloys of zinc, tin, lead and magnesium. 2. At the inner bore, where they are machined off. 3. ω = √(70 × 9.81/0.15) = 67.7 rad/s ≈ 646 rpm. 4. It handles high-melting alloys, complex thin shapes and gives excellent finish with no parting line. 5. Fine silica sand bonded with thermosetting phenolic resin.