The appearance of the final anodised surface is greatly influenced by the mechanical finishing of the basis metal before parts are submitted for treatment at the anodising factory.
Anodising won't hide any pre-existing defects in the material surface, such as scratches, embedded debris or marks introduced during the extrusion, rolling or machining of the base material. Any such imperfections need to be removed by mechanical means such as polishing, linishing, or abrasive blasting, dependent on the desired appearance.
The roughness of the surface finish will affect the optical properties of the final part (whether it appears dull or shiny). Increased surface roughness will cause a more matt appearance. For the final part to have a shiny appearance it is necessary to have a highly-polished surface prior to anodising. If requested, the treatment process can be controlled to maintain a high degree of reflectivity but it should be noted that certain alloys will always develop a matt surface on anodising. See Selection of Aluminium Alloys for Anodising.
Chemical etching is often part of the pre-treatment we carry out before an item is anodised. Etching removes a very thin surface layer of material and is necessary to remove the native oxide film so that the part can be anodised successfully.
The chemical pre-treatment which we give to parts can be varied according to customer requirements. We are only able to make items more matt; we do not have the facility for chemical polishing. Any specific requirements for a matt surface should be requested on the purchase order.
Examples of mechanical finishing techniques which are frequently used to modulate the appearance of aluminium parts include abrasive grinding, polishing/buffing, linishing, scratch brushing or abrasive blasting. If time, money and effort has been expended achieving the desired surface finish it is important to ensure parts are well packaged to minimise likelihood of damage in transit.
Potential Problems to Avoid
- Don't put sticky tape directly on the surface of parts – the residue it leaves behind is not always easily visible and is not removed by our standard cleaning and pre-treatment processes. Residue removal will involve additional cost.
- Avoid fingerprint-corrosion by refraining from handling untreated parts with bare hands.
- If abrasive blasting is used, ensure that this is carried out evenly as inconsistencies introduced by uneven abrasive blast are often accentuated by anodising. Using lower blast pressures, larger diameter blast media or softer blast material may help to alleviate this.
- Ensure swarf is removed from tapped holes as this can be a site of entrapment for processing chemicals.
- It is very important that separate linishing belts are used for aluminium and non-aluminium materials to avoid the introduction of foreign particles embedded in the surface. Contamination from dirty linishing belts may not be visible until after items have been anodised, at which point it shows up as unsightly grey marks or pitting.
- Whilst rumbling parts to de-burr is an economical method, it does not give an attractive surface finish.
- Re-anodising may be less attractive.
More Info and Specialist Advice
Please email us if you would like advice on surface preparation prior to anodising. This page just gives general suggestions and our specialists will be able to provide more in-depth information.
This page is provided for information only, it should not be considered advice and we cannot accept any responsibility or liability for your use of the information on this page. The information on this page is used and relied on at your own risk and you bear the sole responsibility for any outcomes. E&OE.