Cores and core making
Domain 5 - Cores and core making
This domain tests how internal cavities are formed: core sands and binders, core-box types, core prints, venting, baking and support against buoyancy.
Core materials and types
Core sand is high-silica sand with a binder and little clay, because it needs high dry strength and permeability. Binders include core oil, dextrin, sodium silicate for the CO2 process, phenolic resin and furan resin. Cores may be green-sand, dry-sand, oil-sand, shell, CO2, hot-box or cold-box. Core wash or coating improves surface finish and reduces metal penetration.
Design and support
Core prints in the pattern support the core and form its seat. Chaplets are metal supports used when prints are insufficient. Vents in the core let gas escape through the core print. Cores must be collapsible after solidification; otherwise they cause hot tears or cracks. Baking cures oil-sand cores; over-baking makes them friable.
Worked example
A core displaces 500 cm3 of molten iron of density 7.2 g/cm3. Upward buoyant force equals the weight of displaced metal: 500 × 7.2 = 3 600 g, or about 35 N. The core print and chaplets must resist this force, otherwise the core floats and the cavity is displaced. The calculation also shows why large cores need generous prints or anchoring.
Traps
Do not block core vents with wash or sand. A core that is too strong and non-collapsible can crack the casting as it contracts. Moisture in a core causes blows; dry cores thoroughly. Core-box dimensions must allow for core shrinkage. Chaplets must be clean and must fuse with the metal, or they create inclusions.