Moulding sands and mould making

Moulding sand constituents, kinds and properties, standard sand tests (permeability number, grain fineness, moisture) and green-sand mould making.

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Why it matters

Most of the world's iron and steel castings are still poured into sand moulds, and a large share of casting scrap — blowholes, scabs, burn-on, mould-wall movement — is traced back to the sand, not the metal. Knowing what each sand property does, and how the foundry measures it, lets you diagnose a defect and correct the sand mix instead of guessing.

Key ideas

What moulding sand is. A moulding sand is a mixture of a refractory base sand, a binder and water (plus additives).

  • Base sand: usually silica sand (SiO₂), cheap and refractory to about 1700 °C. Zircon, chromite and olivine sands are used where higher refractoriness, conductivity or lower thermal expansion is needed (steel castings, chills).
  • Binder: clay — bentonite (montmorillonite) or fire clay — typically 5–15%. Clay coats the grains and, when wetted, bonds them. Organic binders (oils, cereals, resins) and inorganic ones (sodium silicate, cement) are used for dry sand, cores and no-bake moulds.
  • Water: typically 2–8%; it activates the clay. Too little gives weak sand; too much gives steam, low permeability and blow defects.
  • Additives: sea-coal (coal dust) for cast iron gives a reducing gas film and a smoother skin; cereal or wood flour cushions sand expansion and improves collapsibility; silica flour fills gaps for a fine finish.

Kinds of sand by use. Green sand is moist clay-bonded sand used without drying — cheap and fast, the standard for small and medium iron castings. Dry sand moulds are made of green sand and then baked, giving higher strength and fewer gas problems for medium-to-large castings. Loam sand is about 50% clay, applied wet over a brick framework and shaped with sweeps for very large castings. Facing sand is the fine, rich layer next to the pattern; backing (floor) sand is reused sand that fills the rest of the flask; parting sand is clay-free sand dusted on the parting surface so the cope and drag separate cleanly.

Properties and what controls them.

  • Permeability — ability to let gas and steam escape. Increased by coarser, more uniform, rounded grains; reduced by more clay, more water (beyond the optimum), fines and harder ramming.
  • Strength (green and dry; compression, shear, tensile) — holds the shape against handling and metallostatic pressure. Increases with clay and with water up to an optimum, then falls.
  • Refractoriness — resistance to heat without fusing; set by the base sand.
  • Collapsibility — the mould and core give way as the casting contracts, avoiding hot tears.
  • Flowability — ability to fill pattern detail when rammed.
  • Adhesiveness and cohesiveness — sticking to the flask, sticking to itself.

Several of these pull against each other. Fine sand gives a better surface but lower permeability; more clay gives strength but lowers permeability and refractoriness; harder ramming raises strength and lowers permeability. The foundry engineer picks a balance for each casting.

Sand testing. Standard tests (AFS / IS) use a cylindrical specimen 50.8 mm (2 in) in diameter and 50.8 mm high, made with three rams of a standard rammer. Tests include permeability, green compression strength, moisture (weight loss on drying), clay content (washing out particles finer than 20 µm), grain fineness by sieving, and mould hardness.

Grain fineness number (GFN). A dried, clay-free sample is shaken through a stack of standard sieves. The weight on each sieve is multiplied by a factor (a multiplier tabulated for each sieve, roughly the mesh number of the previous sieve), and GFN is the weighted average. Higher GFN means finer sand. Typical foundry sands lie between about 50 and 90.

Making a green-sand mould. Place the drag half of the pattern on a mould board, sprinkle facing sand, ram the drag full, strike off and vent, roll it over. Place the cope half and the sprue and riser pins, dust parting sand, ram the cope, vent with a wire. Lift the cope, rap and draw the pattern, cut the gates, repair the cavity, set the cores, close the mould with the cope, weight or clamp it and pour. Machine moulding (jolt, squeeze, jolt-squeeze, sand slinger) replaces hand ramming for production; high-pressure flaskless lines make hundreds of moulds an hour.

Formulas

PN = (V × H) / (p × A × t)

  • PN = permeability number (by convention reported without units), V = volume of air passed (cm³), H = specimen height (cm), p = air pressure (g/cm², numerically equal to cm of water column), A = specimen cross-section (cm²), t = time (min). This mixed-unit formula is the standard test convention; do not convert to SI.
  • For the standard specimen (H = 5.08 cm, A = 20.27 cm²) and V = 2000 cm³: PN = 501.3 / (p × t).

GFN = Σ(wᵢ × mᵢ) / Σwᵢ

  • wᵢ = mass retained on sieve i (g), mᵢ = multiplier for that sieve (from the AFS table, given in the question).

Moisture % = (mass before drying − mass after drying) / mass before drying × 100

Clay % = (mass of sample − mass after washing out clay) / mass of sample × 100

Worked examples

Example 1 (standard — permeability). In a standard permeability test, 2000 cm³ of air passes through the standard specimen in 1 min at a pressure of 10 g/cm². Find the permeability number.

  1. A = (π/4) × 5.08² = 20.27 cm².
  2. PN = (V × H) / (p × A × t) = (2000 × 5.08) / (10 × 20.27 × 1).
  3. PN = 10 160 / 202.7 = 50.1.

Answer: PN ≈ 50. If the same sand took 2 min, PN would halve to about 25 — slower air flow means less permeable sand.

Example 2 (GATE level — grain fineness). 200 g of dried, clay-free sand is sieved. Masses retained and their multipliers are: 10 g (×20), 20 g (×30), 40 g (×40), 60 g (×50), 40 g (×70), 20 g (×100), 10 g (×140). Find the GFN and comment.

  1. Σwᵢ = 10 + 20 + 40 + 60 + 40 + 20 + 10 = 200 g.
  2. Σwᵢmᵢ = 200 + 600 + 1600 + 3000 + 2800 + 2000 + 1400 = 11 600.
  3. GFN = 11 600 / 200 = 58.

Answer: GFN = 58 — a medium sand, suitable for general grey-iron work. A fine finish on small castings would need a higher GFN (finer sand) and accepting lower permeability.

Example 3 (moisture). A 50 g green-sand sample weighs 48.2 g after drying at 105–110 °C. Moisture = (50 − 48.2) / 50 × 100 = 3.6%.

Common mistakes

  • Thinking "more water always means more strength". Green strength rises with water only up to an optimum, then falls, and permeability falls with it.
  • Saying fine sand improves permeability. Fine sand improves surface finish but reduces permeability.
  • Converting the permeability formula to SI. The permeability number is defined in the mixed test units (cm³, cm, g/cm², min).
  • Forgetting that time in the permeability formula is in minutes.
  • Confusing loam sand (≈50% clay, for large sweep-made moulds) with green sand (≈5–15% clay).
  • Treating parting sand as a binder-rich sand; it is clay-free so it does not stick.

For GATE PI

Expect one-mark questions matching sand properties to their effect (permeability and blowholes, collapsibility and hot tears, refractoriness and fusion/burn-on), identifying what raises or lowers permeability, and naming the role of additives. Numericals ask for permeability number from test data, grain fineness number from sieve data, and moisture or clay percentages. Practise the permeability formula with the standard specimen until the 501.3/(p·t) shortcut is automatic.

Quick check

  1. What happens to permeability when the clay content of a sand is increased?
  2. Why is facing sand finer and richer than backing sand?
  3. In a standard test, 2000 cm³ passes in 2 min at 5 g/cm². What is PN?
  4. Which property prevents hot tears in a casting?
  5. Which sand is dusted on the parting surface, and why is it clay-free?

Answers: 1. It decreases (clay fills the voids between grains). 2. It is next to the metal and must give a good surface and resist burn-on; backing sand only supports. 3. 501.3 / (5 × 2) ≈ 50. 4. Collapsibility. 5. Parting sand — clay-free so the cope and drag do not stick together.

Interview questions

All Casting, Forming and Joining interview questions

Try answering each one aloud before you open it.

  1. 1.What is moulding sand and what are its main components?Concept

    Moulding sand is a mixture used in the casting process to create moulds. Its main components include silica sand, clay (as a binder), and water. Additional materials like coal dust or other additives may be included to enhance specific properties such as strength or surface finish.

  2. 2.Explain the difference between green sand and dry sand moulding.Concept

    Green sand moulding uses sand that is moist and contains clay and water, which gives it plasticity and strength. Dry sand moulding involves using sand that is dried and baked after the mould is made, which provides better dimensional accuracy and surface finish but is more expensive and time-consuming.

  3. 3.Why is permeability an important property of moulding sand?Application

    Permeability is crucial because it allows gases and steam to escape from the mould during the casting process. If the sand is not permeable enough, trapped gases can cause defects in the casting, such as blowholes or gas porosity.

  4. 4.What role does clay play in moulding sand?Application

    Clay acts as a binder in moulding sand, providing cohesion and strength to the sand particles. This helps the sand retain its shape and withstand the forces during the casting process. The clay content must be balanced to ensure the sand has adequate strength without compromising permeability.

  5. 5.What happens if the moisture content in moulding sand is too high?Application

    Excessive moisture in moulding sand can lead to steam generation during the casting process, which may cause defects like blowholes or gas porosity in the final casting. It can also reduce the strength of the mould, leading to deformation or collapse.

  6. 6.How does the grain size of sand affect the properties of a mould?Application

    The grain size of sand affects the surface finish and permeability of the mould. Finer grains provide a smoother surface finish but may reduce permeability, while coarser grains increase permeability but can result in a rougher surface finish. The choice of grain size depends on the specific requirements of the casting process.

  7. 7.Why is it important to control the temperature of the sand in the moulding process?Application

    Return sand from shake-out can be hot. Hot sand loses moisture unevenly, so the clay is not properly activated, the sand sticks to patterns and gives weak, inconsistent moulds and poor surface finish. Foundries cool return sand (typically to below about 50 °C) in coolers before re-mulling so moisture and strength can be controlled.

  8. 8.Calculate the permeability number of a moulding sand if 2000 cm³ of air passes through the standard AFS specimen (50.8 mm diameter × 50.8 mm high) in 2 minutes at a pressure of 10 g/cm² (10 cm of water).Numerical

    PN = V·H / (p·A·t), with V = 2000 cm³, H = 5.08 cm, A = (π/4)·5.08² = 20.27 cm², p = 10 g/cm² and t = 2 min. PN = 10 160 / (10 × 20.27 × 2) ≈ 25. For the standard specimen and 2000 cm³ this reduces to PN = 501.3/(p·t), a shortcut worth remembering. A PN of 25 is low, typical of fine or heavily rammed sand.

  9. 9.What is the effect of adding coal dust to moulding sand?Application

    Adding coal dust to moulding sand can improve the surface finish of the casting by reducing the likelihood of sand burning onto the casting surface. It also helps in reducing the metal penetration into the sand, which can lead to a smoother surface and fewer defects.

  10. 10.Explain the significance of the AFS (American Foundry Society) grain fineness number in moulding sand.Concept

    The AFS grain fineness number indicates the average size of sand grains in a moulding sand mixture. It is significant because it helps in selecting the appropriate sand for specific casting requirements, balancing surface finish and permeability. A higher AFS number indicates finer sand, which is suitable for detailed castings, while a lower number indicates coarser sand, suitable for larger castings.

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