One thing to note when it comes to the anodising of extruded aluminium profiles is that there are a few key points to be aware of when specifying the process on this type of product.
Choose the Right Alloy Before You Extrude
6000- series alloys (e.g. 6063 and 6082) which are the most common extrusion alloys, produce good results when anodised. Higher copper or silicon content may produce a dull, darker or patchy finish and are best avoided if appearance or coating uniformity is critical. Note that alloy selection is generally decided prior to extrusion and so needs to be considered prior to the profiles being made.
Expect Some Grain Streaking in the Finish
The force of extrusion causes the aluminium to flow through a die in one direction, creating long lines of grain in the part. The anodizing process can then bring out the streaks of these lines on bright or clear finishes. Mechanical pre-treatment, such as brushing or etching, before anodizing can reduce the streaks, but they are rarely removed completely on articles requiring a decorative finish.
Allow for Dimensional Growth on Tight Features
The layer of a sulphuric acid anodising process grows into the part but also outwards from the surface. This outward growth must be allowed for when dimensioning parts with tight tolerance such as slots, holes etc and thin walls. This growth is roughly 50% of the thickness of the layer and for typical coating thicknesses of 15 to25 microns this amounts to an outward growth of 8 to 12 microns per surface.
Fixturing Long Profiles Needs Careful Planning
As long extrusions are generally more difficult to stack than short machined parts, careful consideration must be taken to locate contact points to carry current without marking, and to ensure that the extrusion hangs straight to prevent coating variations down its length. If you are looking for Sulphuric acid anodising, see www.poeton.co.uk/surface-treatments/anodising/sulphuric-acid-anodising.
Clean the Surface Thoroughly Before the Bath
Extruded aluminium profiles often have die lubricant and other marks on the surface. These must be degreased and alkaline etched before anodizing to ensure that the surface is open to form a uniform oxide layer.
Get the specification right from the outset to avoid the chance for ironing out errors in finishing extruded parts which generally are fairly straightforward to anodize and provide a great durable finish.
