Casting defects — identifying defects due to poor venting Which casting defect specifically results from improper venting of the moulding sand, causing trapped gases within the casting?

Introduction / Context:
Gas-related casting defects arise when mould gases cannot escape during metal pouring and solidification. Recognising the defect type helps in diagnosing and correcting sand and gating issues.

Given Data / Assumptions:

• Green-sand moulds with inadequate venting or excessive moisture.
• Molten metal evolves gases and displaces air; permeability must allow their escape.
• Defect observed as rounded or elongated cavities inside the casting.

Concept / Approach:
Blow holes are smooth-walled gas cavities formed when gases become trapped within the solidifying metal. Poor venting, high moisture, or improper binder levels reduce sand permeability. Remedies include improving venting, optimising moisture, using proper gating/risering, and ensuring clean charge materials.

Step-by-Step Solution:
Identify symptom: internal cavities associated with gas entrapment.Relate cause: inadequate venting/permeability → blow holes.Select “blow holes”.

Verification / Alternative check:
Fractography of the defect shows smooth, sometimes oxidised walls typical of gas cavities rather than lack-of-fusion lines.

Why Other Options Are Wrong:

• Cold shuts: caused by premature chilling or two metal streams not fusing.
• Shift: misalignment of mould halves or cores.
• Swell: mould wall yielding due to low strength/over-ramming, not gas entrapment.

Common Pitfalls:
Confusing blow holes with shrinkage porosity; the latter relates to feeding and solidification, not gas venting.

Final Answer:
blow holes