What is Anodised Aluminum?

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Dyeing

Colored iPod Mini cases are dyed following anodization and before thermal sealing

The most common anodizing processes, for example sulfuric acid on aluminium, produce a porous surface which can accept dyes easily. The number of dye colors is almost endless; however, the colors produced tend to vary according to the base alloy. Though some may prefer lighter colors, in practice they may be difficult to produce on certain alloys such as high-silicon casting grades and 2000-series aluminium-copper alloys. Another concern is the "lightfastness" of organic dyestuffs—some colors (reds and blues) are particularly prone to fading. Black dyes and gold produced by inorganic means (ferric ammonium oxalate) are more lightfast. Dyed anodizing is usually sealed to reduce or eliminate dye bleed out.

Alternatively, metal (usually tin) can be electrolytically deposited in the pores of the anodic coating to provide colors that are more lightfast. Metal dye colors range from pale champagne to black. Bronze shades are commonly used for architectural use.

Alternatively the color may be produced integral to the film. This is done during the anodizing process using organic acids mixed with the sulfuric electrolyte and a pulsed current.

Splash effects are created by dying the unsealed porous surface in lighter colors and then splashing darker color dyes onto the surface. Aqueous and solvent based dye mixtures may also be alternately applied since the colored dyes will resist each other and leave spotted effects.

You need to specify hard anodising. It's available in a wide range of colours - see e.g. http://www.alphaanodising.co.uk/ (usual disclaimers).

Reply to Lynlynch and JohnDG

Thanks. On reflection it seems as though the plating shop only knew of basic acid plating/etching. They didn't know about the various finishes - or if they did - they did not tell me.

Regards.

Your casting won't take a good anodised finish because of the silicon content of the alloy used to make it flow easily. This could be as high as 12% & will cause the anodising to either not take at all or be very patchy. Anodising will only work well with a limited range of alloys.

Most appropriately analysed!

I was told an easy way to anodise aluminium is to
plunge it into boiling water. First obtain the finish and colour
required and then to seal it, simply plunge into boiling water.

Hope this helps.

jt.

Hard coat anodizing will be more durable but the color and finish will still be dependent on your casting material and finish. You may want to consider powder coating.