Sulphuric acid anodising on Machined Aluminium Parts
Preparation of machined aluminium parts for anodising includes careful consideration and specification in order to prevent dimensional errors and inadequate adhesion of the anodic oxide layer.
Understand How the Layer Grows
The anodic oxide film grows approximately half of its thickness into the Aluminium and half out from the original surface of the Aluminium. Thus for example a 20µm film will add 10µm to each external dimension. For close tolerance machined parts, therefore, it is necessary to include in the pre-anodising dimensions, the thickness of the anodic film to be produced. This will ensure that after processing, the bores and other holes will be large enough and that the tolerances of the external dimensions will be met.
Choose the Right Alloy Grade
As already mentioned above, the alloy grade has a significant effect on the finish. The 1000, 5000 and 6000 series are good for anodising. They produce clear and consistent coatings. High-copper alloys (2000 series) and high-silicon casting alloys produce patchy, poorly adhered oxide layers and uneven colour. Therefore, when specifying parts for sulphuric acid anodising, it is essential to discuss the alloy grade with the processor before the parts are machined, not after.
Watch Out for Edges and Blind Holes
Sharp edges can create concentrated areas of current during anodizing and cause localized burning or uneven oxide formation. Break off sharp edges before anodizing to prevent damage. Deep recesses such as blind holes and narrow slots can form thin areas where current flow is restricted and coat thickness is less than on open surfaces. Avoid relying on these areas for consistent coating thickness.
Get the Surface Condition Right
Your finished parts could be covered in tool marks, burrs, even in coolant residue. These must be removed before your parts are anodised to ensure good adhesion. A rough turned surface and a smooth ground surface will anodise to different apparent colours and may be mistaken for defects.
Specify Masking Before You Send Parts
Many threaded holes, precision bores and bearing surfaces are masked off to ensure that these high quality machined features are supplied at base metal dimensions, rather than having an anodic layer built up on them. A written specification should be agreed with the processor for the masking of specific surfaces, and the specification of the dimensional tolerance of any masking overlap.
Put the Specification in Writing
By ensuring that a clear written specification is in place regarding the dimensions, alloy, masking and surface finish required before machining commences, time and cost will be saved at each stage.