Casting Processes
Sand Casting, Die Casting, Investment Casting — Solidification, Gating, Defects & GATE Prep
Last Updated: March 2026
Key Takeaways 📌
- Casting = pouring molten metal into a mould cavity and allowing it to solidify into the desired shape.
- Sand casting is the most widely used casting process — versatile, low cost, suitable for almost any metal and any size.
- Chvorinov’s rule: Solidification time t = C(V/A)² — thicker sections solidify last. Risers must have higher V/A than the casting.
- Gating system: Pouring basin → sprue → runner → gate → mould cavity. Designed to fill smoothly without turbulence.
- Common defects: shrinkage, porosity, hot tears, misrun, cold shut — each has specific causes and remedies.
- Die casting (high volume, excellent finish) and investment casting (complex shapes, tight tolerances) are the main alternatives to sand casting.
1. What is Casting?
Casting is a manufacturing process in which molten metal (or other material) is poured into a mould that contains a hollow cavity of the desired shape. The metal solidifies within the mould, and the resulting solid piece — the casting — is then removed. It is one of the oldest manufacturing processes, dating back over 5,000 years.
Casting is uniquely versatile: it can produce parts from a few grams (jewellery, dental implants) to hundreds of tonnes (ship propellers, turbine casings). It can handle extremely complex internal geometries (engine blocks with internal coolant passages) that would be impossible or prohibitively expensive to machine from solid stock.
2. Sand Casting — Step by Step
Sand casting is the most common casting process, accounting for roughly 70% of all castings produced globally. The mould is made from compacted sand mixed with a binder (clay, resin).
- Pattern making: A pattern (replica of the desired casting, slightly oversized for shrinkage allowance) is made from wood, metal, or plastic.
- Mould preparation: The pattern is placed in a flask (box) and sand is packed around it. The mould is typically in two halves — the cope (top) and the drag (bottom).
- Core placement: If internal cavities are needed, sand cores are placed inside the mould before closing.
- Mould assembly: Cope and drag are assembled, creating the complete mould cavity with gating system.
- Pouring: Molten metal is poured through the gating system into the cavity.
- Solidification & cooling: Metal solidifies and cools within the mould.
- Shakeout: The sand mould is broken to remove the casting.
- Cleaning & finishing: Gates, risers, and flash are removed. Surface is cleaned by shot blasting.
Pattern Allowances
| Allowance | Purpose | Typical Value |
|---|---|---|
| Shrinkage | Compensate for metal contraction during solidification | 1–2.5% depending on metal |
| Draft | Taper on vertical surfaces for easy pattern removal from sand | 1–3° for external, 3–5° for internal |
| Machining | Extra material for subsequent machining to final dimensions | 1.5–6 mm depending on size |
| Shake / Rapping | Compensate for mould enlargement when pattern is rapped for removal | Negative allowance (pattern made smaller) |
| Distortion | Compensate for warping in thin, flat, or U-shaped castings | Varies with geometry |
3. Solidification & Chvorinov’s Rule
When molten metal is poured into a mould, solidification begins at the mould walls (where heat is removed fastest) and progresses inward. The last region to solidify is the thermal centre — the thickest part of the casting.
Chvorinov’s Rule
ts = C × (V/A)²
Where:
- ts = solidification time
- V = volume of the casting
- A = surface area of the casting in contact with the mould
- C = mould constant (depends on mould material, metal properties, superheat)
Engineering significance: The ratio V/A is called the modulus of the casting. Sections with larger modulus solidify last. This principle governs riser design — the riser must have a larger modulus than the casting section it feeds, so it solidifies last and can supply liquid metal to compensate for shrinkage.
Directional solidification: The ideal solidification pattern progresses from the thinnest sections (farthest from the riser) towards the riser. This ensures that shrinkage is always compensated by liquid metal from the riser, preventing internal shrinkage cavities.
4. Gating System Design
The gating system channels molten metal from the ladle into the mould cavity. Its components are:
| Component | Function |
|---|---|
| Pouring basin | Funnel-shaped cup that receives metal from the ladle and feeds the sprue |
| Sprue | Vertical channel connecting pouring basin to runner. Tapered to prevent aspiration (air entrainment) |
| Sprue base / well | Cushion at the bottom of the sprue to absorb impact and reduce turbulence |
| Runner | Horizontal channel at the parting line that distributes metal to gates |
| Gate (ingate) | Opening through which metal enters the mould cavity. Controls flow rate and direction |
Gating Ratio
Gating ratio = Sprue area : Runner area : Gate area
Pressurised system: 1 : 0.8 : 0.6 (gate is the choke — controls flow). Used for ferrous metals.
Unpressurised system: 1 : 2 : 4 (sprue is the choke — reduced turbulence). Used for aluminium, copper.
Sprue Design — Continuity
Applying Bernoulli and continuity between the top and bottom of the sprue:
Atop/Abottom = √(hbottom/htop)
The sprue is tapered (narrower at the bottom) to maintain filled flow and prevent aspiration.
5. Riser Design
A riser (also called a feeder) is a reservoir of molten metal attached to the casting. Its purpose is to supply liquid metal to the casting as it shrinks during solidification, preventing shrinkage cavities.
Riser design rules:
- The riser must solidify after the casting section it feeds → riser modulus (V/A) > casting modulus.
- The riser must contain enough liquid metal to compensate for solidification shrinkage (typically 3–7% by volume for metals).
- The riser should be placed at or near the last point to solidify (thermal centre).
- Top risers (open risers) are placed on the cope surface. Side risers (blind risers) are enclosed within the mould.
Caine’s Method (Riser Sizing)
(V/A)riser > (V/A)casting — ensures riser solidifies last.
A common rule of thumb: riser modulus should be at least 1.2× the casting modulus.
6. Die Casting
Die casting uses a permanent metal mould (die) and forces molten metal into the cavity under high pressure (7–350 MPa). It produces high-volume parts with excellent dimensional accuracy and surface finish.
| Type | Mechanism | Metals | Pressure |
|---|---|---|---|
| Hot chamber | Injection mechanism submerged in molten metal | Zinc, tin, lead (low melting point) | 7–35 MPa |
| Cold chamber | Metal ladled into injection chamber each cycle | Aluminium, magnesium, copper | 20–350 MPa |
Advantages: excellent surface finish (Ra 0.8–3.2 μm), tight tolerances (±0.1 mm), high production rates (100–2000 parts/hour), thin walls possible (0.5–1 mm). Limitations: high die cost (only for large volumes), limited to non-ferrous metals (mostly), porosity from trapped air.
7. Investment Casting (Lost Wax)
Investment casting uses a wax pattern coated with ceramic slurry. The wax is melted out, leaving a ceramic shell mould. Molten metal is poured into this shell, producing parts with very complex geometry and excellent surface finish.
Steps: wax pattern → ceramic shell building (dipping + stuccoing, 6–10 layers) → dewaxing (wax melted/burned out) → firing (ceramic hardened) → pouring → shell removal → finishing.
Advantages: any castable metal (including superalloys), complex geometries with undercuts and thin walls, excellent surface finish (Ra 1.6–3.2 μm), tight tolerances. Limitations: expensive (pattern for each casting), slow process, limited to smaller parts (typically < 50 kg).
Applications: turbine blades (aerospace), medical implants, jewellery, golf club heads, firearm components.
8. Casting Processes — Comparison
| Feature | Sand Casting | Die Casting | Investment Casting |
|---|---|---|---|
| Mould material | Sand (expendable) | Metal die (permanent) | Ceramic shell (expendable) |
| Metals | Almost all | Non-ferrous (Al, Zn, Mg) | All, including superalloys |
| Part size | Small to very large | Small to medium | Small to medium |
| Dimensional accuracy | ±0.5–2 mm | ±0.1 mm | ±0.1–0.25 mm |
| Surface finish (Ra) | 6–25 μm | 0.8–3.2 μm | 1.6–3.2 μm |
| Production volume | 1 to 1,000s | 10,000+ (high volume) | 100–10,000 |
| Tooling cost | Low | Very high | Medium |
| Complexity | Moderate | Moderate (no undercuts) | Very high (undercuts OK) |
9. Common Casting Defects
| Defect | Cause | Remedy |
|---|---|---|
| Shrinkage cavity | Insufficient feed metal during solidification | Proper riser design, directional solidification, chills |
| Porosity (gas) | Dissolved gas released during solidification | Degassing, proper venting, controlled pouring temperature |
| Hot tears/cracks | Thermal stress during solidification; mould restraint | Uniform section thickness, proper allowances, collapsible cores |
| Misrun | Metal solidifies before filling the mould completely | Increase pouring temperature, improve gating, reduce section thickness constraints |
| Cold shut | Two metal streams meet but do not fuse properly | Increase fluidity, improve gate location, raise pouring temperature |
| Sand inclusion | Sand particles break from mould wall and get trapped | Better sand properties, proper ramming, mould coating |
| Metal penetration | Liquid metal penetrates between sand grains | Finer sand, mould wash, reduce pouring temperature |
| Blow holes | Gas pockets near the surface | Improve mould permeability, proper venting, reduce moisture |
10. Common Mistakes Students Make
- Confusing shrinkage allowance with machining allowance: Shrinkage allowance makes the pattern larger to compensate for metal contraction. Machining allowance adds extra material for later machining. Both make the pattern larger than the final part, but for different reasons.
- Using V/A of the casting for the riser: The riser’s V/A must be GREATER than the casting’s V/A — not equal. If riser V/A = casting V/A, they solidify at the same time, and the riser cannot feed the casting.
- Forgetting that Chvorinov’s rule uses (V/A)², not V/A: Solidification time is proportional to the SQUARE of the modulus. Doubling V/A quadruples solidification time.
- Mixing up hot chamber and cold chamber die casting: Hot chamber = injection mechanism in the melt (low MP metals only). Cold chamber = metal ladled into the chamber (for aluminium and other higher MP metals).
- Not recognising that draft allowance is for pattern removal: Draft is a slight taper on vertical surfaces so the pattern can be withdrawn from the sand without damaging the mould. It is not related to shrinkage or machining.
11. Frequently Asked Questions
What is Chvorinov’s rule?
Chvorinov’s rule states that solidification time is proportional to the square of the volume-to-surface-area ratio: t = C(V/A)². This means thicker, bulkier sections take much longer to solidify than thin sections. It is the fundamental principle for riser design — the riser must have a larger V/A than the casting to ensure it solidifies last and can feed liquid metal to compensate for shrinkage.
What are the most common casting defects?
The most common defects are shrinkage cavities (from insufficient riser feeding), porosity (trapped gas), hot tears (solidification stresses), misruns (metal solidifies before filling the mould), cold shuts (two metal streams don’t fuse), and sand inclusions. Each has specific causes related to mould design, pouring conditions, or metal properties, and specific remedies.