A rejected casting is usually the visible result of an earlier process decision. The fastest way to reduce rejection is to connect the defect to its mechanism and then correct the upstream step.
Surface roughness and burn-on
Surface roughness can be influenced by the prime coat, stucco size, shell drying and firing cycle. A stable prime coat and controlled shell build help preserve detail and reduce the amount of finishing required after shakeout.
Gas porosity and pinholes
Gas-related defects may be linked to shell moisture, incomplete burnout, melt cleanliness or turbulence during pouring. The investigation should compare the defect location with the shell condition and the filling route, not only the final appearance.
Shrinkage porosity
Isolated heavy sections and abrupt changes in wall thickness can create feeding challenges. Better fillet transitions, more uniform sections and an appropriate gating and risering design can improve solidification control.
Hot tearing and shell cracking
Hot tearing is often related to restrained contraction, while shell cracking may originate during dewaxing, drying or firing. Recording when the problem first appears is essential because a crack found after pouring may have started much earlier.
A useful corrective-action process
- Photograph and classify the defect.
- Record alloy, shell batch, firing and pouring conditions.
- Compare the location with the drawing geometry.
- Change one major variable at a time.
- Verify the correction with a controlled sample batch.
Process records and clear acceptance criteria make defect reduction repeatable rather than dependent on guesswork.