Moulding sands: composition, properties, testing and preparation
Domain 2 - Moulding sands: composition, properties, testing and preparation
This domain examines what makes a moulding sand work: the silica base, clay binder, water and additives, the properties tested in the sand laboratory, and how mulling, tempering and reclamation change those properties.
Composition and properties
Silica sand (SiO2) provides refractoriness. Bentonite clay bonds the grains; water activates the clay. Additives include coal dust or sea coal for surface finish, sawdust and wood flour for collapsibility, and dextrin or molasses for green strength. Key properties are green strength, dry strength, permeability, refractoriness, collapsibility, flowability and plasticity. Facing sand contacts the pattern; backing sand fills the flask.
Testing and preparation
- AFS grain fineness number from sieve analysis: finer sand gives smoother surfaces but lower permeability.
- Clay content by washing; moisture by drying; loss on ignition for carbonaceous additives.
- Permeability number from 2 000 cm3 air through a standard specimen: P = VH/(pAt), with H = 5.08 cm, A = 20.27 cm2, p = 10 g/cm2 and t in minutes.
- Green compressive strength on a sand testing machine. Mulling coats clay evenly on grains; tempering distributes moisture; aeration and cooling prepare reclaimed sand.
Worked example
A permeability test passes 2 000 cm3 in 40 s. Convert to minutes: 40/60 = 0.667 min. P = (2 000 × 5.08)/(20.27 × 10 × 0.667) = 10 160/135.2 ≈ 75. A value near 75 is typical of a medium-permeability green sand. If the same sand is over-mulled with extra clay, strength rises but permeability falls, so vents or lower clay content may be needed.
Traps
More water does not always mean more strength; excess water causes blows and pin holes. Very fine sand improves finish but can cause gas defects. Do not confuse green strength (moist) with dry strength. Reclaimed sand must be cooled and tested before reuse.