Casting defects — identification of a “scab” on the surface A scab is a casting defect that appears as sand patches or rough, raised, thin layers on the casting surface, often on the upper faces. Is this description accurate?

Introduction / Context:
Recognizing surface defects helps improve moulding practice and gating design. A “scab” is a common surface defect that indicates mould sand movement or localized metal penetration issues during pouring.

Given Data / Assumptions:

• Scabs typically occur on horizontal or gently inclined surfaces.
• Causes include sand expansion, weak mould surfaces, or dirt/loose sand particles.
• Distinguish scabs from other defects such as cold shuts, misruns, and blowholes.

Concept / Approach:
A scab presents as a thin, brittle, rough layer of metal adhering to but partly separated from the casting, sometimes with sand embedded beneath. It occurs where mould surface flakes or lifts and molten metal flows underneath, creating a raised patch or blister-like region. This matches the description of “sand patches on the upper surface”.

Step-by-Step Solution:
Observe defective area: rough, scaly patch with possible sand inclusion.Relate to root cause: inadequate mould strength, excessive ramming, poor venting, or high metal head causing erosion.Confirm that the observed feature is a scab rather than a cold shut (which is a lap line from two meeting streams) or a misrun (incomplete fill).

Verification / Alternative check:
Foundry defect atlases depict scabs as raised patches with sand embedded; preventive measures include better sand control, facing sand improvement, and proper venting.

Why Other Options Are Wrong:

• False: contradicts standard definition.
• Non-ferrous only: scabs occur in ferrous and non-ferrous alloys.
• Cold shut: shows a smooth lap line, not a rough sandy patch.

Common Pitfalls:
Confusing metal penetration or burn-on with scabs; penetration is uniform roughness, while scabs are localized patches.

Final Answer:
True