Gating and feeding (risering) systems
Domain 6 - Gating and feeding (risering) systems
This domain tests how liquid metal is introduced and fed: gating elements, gating ratios, flow behaviour, riser design and the use of chills and exothermic materials.
Gating system
The system comprises a pouring basin, sprue, sprue well, runner, slag trap and ingates. A tapered sprue avoids air aspiration. Top gating fills quickly but creates turbulence; bottom gating is quieter but has a hot top and slower fill. A gating ratio is written sprue:runner:ingate. A pressurised system, such as 1:2:1, chokes at the ingate; a non-pressurised system, such as 1:4:4, chokes at the sprue and reduces velocity.
Risers and feeding
A riser feeds shrinkage during solidification. It must be placed at the hottest, thickest section and must solidify after the casting. Chvorinov’s rule gives solidification time t = k(V/A)2. A common rule is riser modulus = 1.2 × casting modulus, where modulus = volume/surface area. Chills speed local cooling; exothermic sleeves keep risers hot; padding improves directional solidification.
Worked example
A gating ratio is 1:2:2 and the sprue area is 6 cm2. Runner area = 2 × 6 = 12 cm2. Total ingate area = 2 × 6 = 12 cm2. If two ingates are used, each is 6 cm2. The ratio tells you only relative areas; the choke controls filling time.
Traps
A riser placed on a thin section freezes first and cannot feed. Increasing pouring temperature does not replace a poor riser. Bottom gating is not automatically defect-free; it can produce cold shuts if filling is too slow. Do not confuse gating ratio with sand mix ratio.