8D Case Study / Assembly Quality
ZAMAK 3 Pearl Chrome Die Casting 8D Case Study: Assembly Step Interference from Local Excess Material
A sanitized 8D case for a ZAMAK 3 die cast and hexavalent pearl chrome plated part, showing how a suspected dimensional issue was traced to local excess material, mold erosion and fitting interference with the mating plastic part.
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.
Need die casting assembly interference review?
Send drawings, samples, fitting criteria, plating requirements and mating-part information. XSD can support dimensional review, mold-risk analysis and corrective-action planning.
8D 案例学习 / 装配质量
ZAMAK 3 珍珠铬压铸件 8D 案例：局部多料导致装配台阶干涉
本脱敏 8D 案例针对 ZAMAK 3 锌合金压铸并电镀六价珍珠铬的产品，说明如何从尺寸争议追溯到局部多料、模具冲蚀以及与塑胶件装配干涉的真实风险点。
脱敏案例范围
| 项目 | 脱敏工程记录 |
|---|---|
| 产品类型 | ZAMAK 3 锌合金压铸装饰类钥匙扣/钥匙挂件，与塑胶件配合装配。 |
| 表面处理 | 压铸、去水口、去毛刺、钻孔攻牙、打磨抛光后，进行六价珍珠铬电镀。 |
| 问题信号 | 装配后局部台阶凸出，用手触摸有刮手感；初期关注点集中在两个关键尺寸是否与台阶问题有关。 |
| 脱敏说明 | 本文已删除客户名称、供应商名称、公司名称、人员姓名和产品料号，仅保留工程分析逻辑。 |
8D 排查路线
临时围堵与挑选证据
| 动作 | 结果 |
|---|---|
| 客户侧挑出不良品 | 更换配合塑胶件并重新实配后，仅少量产品仍出现台阶，说明问题不能只用金属件两个尺寸解释。 |
| 退回批次实配检验 | 退回批次使用塑胶件进行实际适配检测，适配合格品按结果放行。 |
| 内部库存检查 | 在制库存通过实配方式全检确认合格。 |
| 尺寸样本分析 | 对 200 件适配 OK 的产品测量后发现，部分尺寸虽超出当前图纸上限，但实际装配无台阶。 |
原因分析
- 最初关注的两个尺寸与台阶刮手问题没有表现出强直接关联；部分超出当前图纸上限的产品仍可实配 OK。
- 某一尺寸靠近下偏差时反而更容易出现台阶风险，因此不建议简单要求该尺寸尽量走下公差。
- 对仍然不良的样件仔细观察后，发现局部存在多料并与塑胶件干涉；将多料位置修挫后重新实配，台阶消失。
- 继续往前排查，发现毛坯件同一位置存在不同程度多料；该位置不属于正常外观打磨抛光区域，因此抛光无法自动修复。
- 检查模具后发现，对应位置存在冲蚀/磨损，导致压铸件圆弧角局部翘起或形成轻微凸包，最终与塑胶件配合时形成断差台阶。
真因结论
本案例中，台阶刮手的主要原因是模具对应位置冲蚀/磨损，引起非抛光圆弧区域局部多料或翘起，装配时与塑胶件发生干涉。尺寸超差是重要排查线索，但不是造成台阶凸出的直接真因。
改善验证
| 验证项目 | 结果 |
|---|---|
| 局部多料修挫 | 将原不良样件干涉部位修挫后重新实配，台阶消失，实配 OK。 |
| 模具维修 | 对模具冲蚀位置进行维修和省模，使对应表面恢复光顺，避免毛坯局部翘起。 |
| 修模后试产 | 修模后验证 50 件，实配结果合格。 |
| 尺寸复核 | 修模后在更宽的尺寸分布范围内仍可实配 OK，进一步说明装配尺寸链和局部干涉控制比单独两个尺寸更关键。 |
永久纠正措施
- 对模具冲蚀/磨损区域进行维修维护，避免毛坯在圆弧角产生局部翘起或多料。
- 抛光后增加仿形治具或轮廓检具，专门检查容易被外观检验忽略的干涉区域。
- 该装配界面采用实配确认，尤其在配合塑胶件存在缩水波动时，不只依赖单一金属件尺寸判定。
- 同步关注塑胶件缩水稳定性，把塑胶件尺寸、金属件尺寸和台阶外观一起纳入装配尺寸链管理。
- 对两个争议尺寸进行图纸公差评审，但应以尺寸分布、实配证据和客户允收标准一致为前提。
- 更新检验关注点，避免只确认前期已发生过的问题，而忽略角部凸出、台阶不平等新的装配外观风险。
案例启示
ZAMAK 3 珍珠铬压铸装配件出现台阶或刮手感时，不能只盯住图纸上最显眼的几个尺寸。可靠的 8D 路线应同时结合尺寸数据、实配结果、塑胶件缩水、模具磨损状态和仿形治具检测，才能避免误判真因。
需要压铸装配干涉问题评审？
请提供图纸、样件、适配标准、电镀要求和配合件信息。XSD 可协助进行尺寸评审、模具风险分析和纠正措施规划。