Blowholes and a Misrun Rib in a 6 kg Grey Cast Iron Bracket
A foundry pours a grey cast iron bracket weighing 6 kg in a green sand mould. The pattern is a single piece with a 4 mm thick strengthening rib. Sand tests show 8% moisture and low permeability; the sprue is 18 mm diameter, the runner 12 mm, and there are two ingates of 8 mm. Pouring temperature is 1360 °C and the ladle takes about 55 seconds to fill the mould. Inspection finds smooth round holes with bright inner surfaces near the top of the casting, and the 4 mm rib is incompletely filled at its far end. The moulder wants to cure both defects by raising the pouring temperature to 1450 °C.
1. Identify the two defects and distinguish their appearance from shrinkage porosity and a cold shut.
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The smooth, round, bright holes near the top surface are blowholes. Gas from steam and mould gases has been trapped and has formed rounded cavities with clean walls. The incomplete 4 mm rib is a misrun: the metal chilled before it could fill the thin far end, leaving rounded, smooth-edged discontinuities.
Shrinkage porosity is rough, jagged and usually internal or at a hot spot, not a smooth round hole at the top. A cold shut is a seam where two metal streams met but did not fuse; it has rounded edges and lies along a flow path. The location and appearance therefore separate gas holes from shrinkage and from a cold shut.
2. Evaluate the moulder's plan to raise the pouring temperature to 1450 °C. What will it fix and what will it make worse?
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Raising superheat will improve fluidity and may help the 4 mm rib fill, so it could reduce the misrun. But it will not remove the gas source: 8% moisture and low permeability are already generating steam, and a hotter pour increases gas absorption and sand expansion. The blowholes may become worse, and the casting may also show penetration and shrinkage problems.
The better first move is to reduce sand moisture to about 5–6%, improve permeability and venting, and shorten the 55 s pouring time. Raise temperature only moderately, for example to about 1400 °C, after the sand and gating are corrected. Temperature alone treats the symptom, not the cause.
3. The sprue is 18 mm diameter and there are two 8 mm ingates. Show by area calculation why the ingates are the choke, and propose a better gating ratio.
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Sprue area = π/4 × 18² = 254 mm². Two 8 mm ingates give 2 × π/4 × 8² = 100.5 mm², which is less than half the sprue area. The ingates are therefore the smallest opening and control the fill: metal accelerates through them, jets into the mould, loses heat, and the thin rib misruns.
Use a less pressurised system. Make the total ingate area at least equal to the sprue area, for example two ingates of about 13 mm diameter (2 × 133 = 266 mm²), with a runner of about 25 mm diameter (491 mm²). A ratio near 1:2:2 (sprue:runner:ingate) reduces jet velocity. Add a pouring basin and vents so that gas can escape instead of forming blowholes.