Anodized Housing Color Mismatch Across Two Lots
A contract manufacturer supplies machined 6061-T6 aluminum housings with a Type II clear anodize and a dyed black finish. Cosmetic spec requires a visually uniform black across the assembly, judged under D65 lighting. Lot A (500 pcs) and Lot B (500 pcs) are assembled into the same product. Field returns report that Lot B appears noticeably more brown/less deep-black than Lot A when side by side. Dimensional inspection passes on both lots. The anodizer's process sheet shows Lot A dyed at 70°F for 15 min and Lot B at 82°F for 12 min, with the same dye bath concentration. Sealing was done in deionized water at 200°F for both. The customer will not accept a mixed-lot shipment but will accept a single consistent lot.
1. Is this a dimensional, material, or process-color problem? Explain the mechanism.
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It is a process-color problem, not dimensional or base-material. Dimensions pass, and both lots are 6061-T6 from the same alloy family. The anodize dye uptake depends on coating porosity and dye-bath kinetics. Lot B was dyed at a higher temperature (82°F vs 70°F) for a shorter time (12 vs 15 min). Higher dye-bath temperature can increase dye absorption rate but also shifts shade and can cause the dye to strike differently; shorter dwell reduces total color depth. The combination produces a lighter, browner black. Because the base alloy and anodize type are the same, the difference is attributable to the dyeing parameters (temperature/time), which are the controllable variables. Sealing appears equivalent, so it is not the primary cause - though seal quality can affect final appearance and should be confirmed.
2. What disposition options exist, and which do you recommend given the customer's constraint?
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Options: (a) ship Lot A only and hold Lot B; (b) re-anodize/strip and re-dye Lot B to match Lot A; (c) sort Lot B by visual shade into matched sub-lots; (d) obtain a customer concession for a single-lot shipment of the better-matching lot.
Recommendation: Ship a single consistent lot. Since Lot A is the reference shade, ship Lot A and hold Lot B pending rework. Re-dyeing Lot B is feasible but requires stripping the existing coating (which removes base material and may affect dimensions on tight-tolerance features) - verify wall thickness and critical dimensions afterward. If rework is too costly, sort Lot B into matched groups and ship only as homogeneous lots, never mixed. Do not mix A and B in one assembly. Document the root cause (dye temperature/time drift) and add a dye-bath temperature/time control to the process sheet before the next run.
3. What process controls would prevent recurrence?
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- Lock dye parameters: specify and record dye-bath temperature (e.g., 70°F ±2°F) and dwell time (15 min ±1 min) on the traveler; do not allow ad-hoc changes.
- Bath chemistry monitoring: track dye concentration, pH, and contamination on a schedule; temperature and time interact with concentration.
- Color standards: maintain a physical master panel per shade and require a visual match under D65 before lot release.
- Seal verification: confirm seal quality (e.g., dye-stain or impedance test) since sealing affects final appearance and corrosion resistance.
- Lot traceability: record anodize lot, dye batch, and operator; segregate lots so mixed shipments cannot occur.
- First-article color sign-off at the start of each lot and a periodic in-process check.