Finishes

As well as the organic coatings normally associated with profiled metal claddings, such as Plastisol PVC finishes and PVF2 coatings, manufacturers will offer a number of alternative finishes, such as:

- electrostatic powder coating;

- anodizing;

- thin premium paint coatings;

- vitreous enamelling;

- stainless steel.

Electrostatic powder coating

Powder coating is the application of thermo-hardening polyester/polyurethane within an airoven.This process has to be carried out in the factory, and it is therefore necessary to coat matching flashings and trims. Cured coating thickness should not be less than 75 pm.

Anodizing

Advantage can be taken of the oxidation characteristics of aluminium to provide an anodized aluminium finish (Fig. 6.32).

There are essentially three types of anodizing. The first two are natural anodizing (silver) and two-stage anodizing (bronze colours - the best known example in the UK is Anolok). Both these types of anodizing use sulphuric acid electrolyte to produce an initial silver-coloured anodic film. In the case of silver anodizing this is then sealed. In two-stage anodizing, the film is dyed in a second stage and sealed to give the required shade.

Sulphuric acid produces a relatively coarsegrained pore structure, which is not as resistant to abrasion as the structure produced by integral anodizing (golds, bronzes, greys and blacks). This is a one-stage process in which the colour is produced by the alloying constituents in the parent metal combined with the action of proprietary acids (e.g. Kalcolor, Duranodic, Calanodic) and then sealed. This produces a dense anodic film with superior weathering characteristics and resistance to abrasion. Thus, although silver anodizing is produced in one stage by the action of electrolyte on the metal, it does not have the film density of the true integral anodizing using sulphosalicylic type acids, and it is normally recommended that this type of anodizing should be washed down more frequently than the other types.

Anodizing should be carried out using alloys specifically suited for the process according to BS 3987:1974. The minimum required anodic film thickness is 25 pm for external applications.

The European Wrought Aluminium Association (EWAA, 1978) specification for anodizing types includes a terminology of aluminium anodizing procedures. Anodized aluminium panels will need washing down regularly to avoid pitting of the surface, and thus provision of cleaning rails and cradles should be allowed for

One difficulty is that it is extremely difficult to match the colour of anodizing, which is dependent on the time the panel is in the electrolyte and the type of alloy used. Although it is possible to agree upper and lower limits of colour variation with the manufacturer;

6.32 Typical anodizing bath.

6.33 Panels for Gatwick Airport on Duranar coating line.

6.33 Panels for Gatwick Airport on Duranar coating line.

6.34 Patera building systems using panels 3.6 m x 1.2 m (courtesy of Patera Products Ltd).

some variation must inevitably be accepted, depending upon the degree of quality control in the factory.

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