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Powder Coating Process

Powder coating involves the electrostatic application of finely ground dry paint film to a specially prepared aluminium surface. The subsequent heating of this film allows the powder to melt, flow out and cure to a tough smooth finish.

The powder coating process involves three major stages, which are explained in detail in this section:

1. Chemical processing (pre-treatment)
2. Electrostatic application of powder
3. Curing

1. Chemical processing (pre-treatment)

Before applying the powder coating, the extruded aluminium section must be chemically cleaned (but not etched as in anodising). Cleaning is carried out via nine long (6.5m) tanks containing chemicals.

The chemical processing line (pre-treatment) has three stages – chemical cleaning, conversion coating and drying. Each chemical stage except one, which uses deionised water, is followed by rinsing in town supply water.

Chemical Cleaning

The chemical cleaning processes consist of degreasing (cleaning), chemical cleaning (caustic) and desmut (deoxidise). Once completed, the metal is chemically clean, with essentially the same surface smoothness created at extrusion.

Conversion Coating

A special solution 'converts' the aluminium surface to an iridescent yellow chemical film. This film layer is integral with the metal, promotes strong adhesion of the paint film and helps prevent corrosion.

Rinse tanks remove residues from the surface and then deionised water is used as a final rinse. Using deionised water eliminates the risk of town water supply impurities becoming trapped between the metal surface and the paint film. This would affect paint adhesion and increase the risk of corrosion.

Drying (Oven)

An oven completes the 'cure' of the conversion coating and provides dry metal for painting.

2. Electrostatic application of powder

Once clean and dry, the aluminium sections are loaded onto an overhead conveyor system and moved into a 'paint booth'.

Special 'guns' then apply paint powder to the extruded aluminium section. The primary guns are automatic, with hand guns being used to coat difficult areas of the metal.

Each powder particle is electrically charged as it leaves the gun and is attracted to the earthed aluminium section. The attraction is strong enough to ensure that the powder adheres during the curing process. This type of application is called electrostatic application.

The electrostatic attraction is strong enough to ensure that the faces of the extruded section not in direct line with the gun are coated by back attraction (except for certain deep recesses). So powder waste by over-spraying is minimised.

The overhead conveyor system moves the section through the paint booth and out to a long oven, which completes the painting process by 'curing' the particles.

3. Curing

The oven heats the metal to about 200°C. The high temperature first melts the powder particles, allowing them to flow together to form a film. It then cures the resin system in the film to form a smooth surface.

Other additives within the powder formulation cause them to be attracted or anchored to the conversion coating, forming a cohesive structure.
After the curing oven, sections are transported to a cooling station, inspected, packed and despatched.