ZAMAK 3 Pearl Chrome Die Casting 8D Case Study: Assembly Step Interference from Local Excess Material
ZAMAK 3 Pearl Chrome Die Casting 8D Case Study: Assembly Step Interference from Local Excess Material: material behavior, mold control, plating risk, dimensional verification,.
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ZAMAK 3 Pearl Chrome Die Casting 8D Case Study: Assembly Step Interference from Local Excess Material: material behavior, mold control, plating risk, dimensional verification,.
Sanitized Case Scope
| Item | Sanitized Engineering Record |
|---|---|
| Product type | ZAMAK 3 zinc alloy die cast decorative keychain/key-holder component assembled with a mating plastic part. |
| Surface finish | Hexavalent pearl chrome plating after degating, deburring, drilling and tapping, grinding and polishing. |
| Problem signal | Assembly step protrusion could be felt by hand after fitting; the initial concern was whether two key dimensions were related to the step issue. |
| Confidentiality note | Customer name, supplier name, company name, personnel names and product part numbers have been removed from this case study. |
8D Investigation Route
Containment and Sorting Evidence
| Action | Result |
|---|---|
| Customer-side sorted NG parts | After replacing and refitting the mating plastic part, only a small number still showed step protrusion, indicating the issue was not explained by metal-part dimensions alone. |
| Returned batch refitting inspection | A returned batch was checked by actual plastic-part fitting; conforming pieces were released after fitting confirmation. |
| Internal stock inspection | In-process inventory was checked by actual fitting and confirmed acceptable. |
| Dimensional sample | A 200-piece sample from acceptable fitting parts showed that some dimensions exceeded current drawing limits but still assembled without step protrusion. |
Root Cause Analysis
- The suspected dimensions did not show a strong direct relationship with the assembly step complaint. Some parts above the current drawing upper limits still fitted acceptably.
- Parts close to the lower side of one dimension were more likely to show fitting step risk, so simply forcing the dimension toward the lower tolerance side was not recommended.
- For the remaining step NG parts, local excess material was found at the interference area. After filing the excess material, the same parts were refitted and the step disappeared.
- Upstream checking found that rough castings had different levels of local excess material at the same area. Polishing did not remove it because the location was not part of the normal appearance-polishing surface.
- Mold inspection found erosion/wear at the corresponding mold position. This created local lift or bulge at the die cast arc corner, causing interference with the mating plastic part.
True Cause Statement
The assembly step issue was primarily caused by local excess material from mold erosion/wear at a non-polished arc area. The raised area interfered with the mating plastic component during fitting. The dimensional out-of-spec discussion was a useful investigation input, but it was not the direct root cause of the step protrusion.
Verification After Correction
| Verification item | Outcome |
|---|---|
| Local excess material removal | The originally NG parts were filed at the interference area, then refitted; the step protrusion disappeared. |
| Mold repair | The corresponding mold area was repaired and polished to remove the erosion-related surface condition. |
| Post-repair trial | 50 pieces were verified after mold correction and fitting result was acceptable. |
| Dimension review | Post-repair fitting remained acceptable across a wider observed dimension range, supporting the conclusion that assembly stack-up and local interference control were more important than the original two dimensions alone. |
Permanent Corrective Actions
- Repair and maintain the eroded mold area so the rough casting no longer forms a local lifted arc or excess material.
- Add a profile fixture or contour gauge after polishing to check the non-obvious interference area before final assembly.
- Use actual fitting confirmation for this assembly interface, especially when the mating plastic part may have shrinkage variation.
- Review plastic-part shrinkage stability together with the metal-part tolerance, instead of judging the problem from the metal part alone.
- Review drawing tolerances for the two discussed dimensions only after dimensional distribution, fitting evidence and customer acceptance criteria are aligned.
- Update inspection focus so that future checks cover both previous functional issues and new corner/step protrusion risks.
Case Takeaway
For pearl chrome plated ZAMAK 3 die cast assemblies, a visible or touchable step can come from a small local interference area rather than from the most obvious drawing dimensions. A reliable 8D route should combine dimensional data, actual fitting, mating plastic shrinkage review, mold wear inspection and fixture-based contour control.
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