Why your second anodised batch does not match the first
A customer sends a Pantone number, approves a photo of the first batch, and then rejects the second batch because it is "a different red". Both batches were made to the same instruction. The instruction was the problem.
Anodising is not paint
Paint sits on the surface at a fixed film thickness and covers what is underneath. Anodising grows an oxide layer into the aluminium and dyes it. The colour you see is the dye trapped in that layer, and the layer thickness depends on the alloy, the current density, the bath temperature and the time.
Two consequences. First, the same dye on 6061 and 7075 gives two different colours, because the alloys anodise differently. Second, the same alloy from a different heat lot gives a slightly different colour, because the alloying elements that affect the oxide are not identical between lots.
What actually controls the match
- Alloy and temper. 6061-T6 is the most repeatable. 7075 shifts noticeably and deep colours show it first.
- Oxide thickness. A 10 µm decorative layer and a 25 µm hard layer take the same dye to different depths and read as different colours.
- Dye concentration and bath life. A bath that has run for weeks dyes differently from a fresh one.
- Surface finish before anodising. A bead-blasted surface scatters light and reads lighter than a polished one in the same dye.
The approval step that prevents the argument
Approve a physical coupon, not a screen. The coupon should be made from the same alloy, with the same surface preparation, at the same oxide thickness as the production parts, and it should be kept. When the second batch is questioned, the coupon is the reference, not a photograph taken under unknown light.
We send a coupon before the run on anything where colour is a selling point — snus cans, bike caps, stems. It costs a few days and it removes the most common rejection reason in anodised work.
If the colour must match a painted part
Say so at the quotation stage. Matching anodised aluminium to a painted or powder-coated surface is possible within a Delta-E that a trained eye will still see under daylight. If the parts sit next to each other on a shelf, plan for a visible difference or move both parts to the same finish.