The distinctive appearance of Hard Anodised Aluminium is key information for any person specifying or checking finished parts.
Natural Colour at Different Thicknesses
The colour of a natural, hard anodised surface varies with the thickness of the coating. Thin anodising of approximately 25 microns produces pale grey colours. Medium thickness anodising, within the range 50-75 microns, can produce a range of bronze colours through to dark grey and at the higher end of the range a very dark almost black colour.
Dyeing Is Largely Impractical
Standard anodising takes dye well as the pore structure of the anodised layer is open and receptive. However, the hard anodising process produces a much denser oxide layer and therefore limited dye absorption. Although black dye can be used to produce a dark surface, in most instances the resulting colour will be poor and in addition the underlying grey or bronze base tone will be obvious. Therefore for decorative colour, standard anodising is generally the preferred process.
How Alloy Composition Affects Appearance
For more on hard anodising, take a look at www.poeton.co.uk/surface-treatments/anodising/hard-anodising/.
Alloy grade greatly affects the appearance of the hard anodised surface. High-copper alloys (2000 series) produce a dark, uneven colour whilst 6000 series alloys produce a consistent grey. Silicon-rich casting alloys can even produce a mottled or patchy appearance.
Surface Texture and Sheen
The surface of a hard anodised surface is generally dull (matte finish) and will typically be slightly rougher than the parent material. This surface quality can be specified using the Ra value (surface roughness) for both the parent material and coated surfaces (drawing requirement).
Masking and Finish Boundaries
Where appropriate, only part of a component may require hard anodising and so be masked from the remainder. A clear boundary line will exist across the masking line where the anodised surface will meet the untreated surface. This line will be clean but obvious in appearance, hence the need to discuss with the processor at the quoting stage where on the part this line is to be located.
