05 November 2010

Aluminium Cars - Paint Finishing Processes

Aluminium Car Finishes
Because of the nature of aluminium and in order to achieve necessary stiffness requirements in the finished condition, there has been a need to develop new aluminium alloys that can be heat treated after assembly. This particularly applies to sheet material that is typically pressed in its softer T4 condition with the intention of heat treating it as it passes through a paint shop.
Ref. Whiteley, F (1994)

With respect to the painting process itself, Honda discovered during the development of the aluminium bodied NSX that the appearance of metallic paint is affected largely by the type and alignment of the aluminium pigment in the basecoat. One way of improving the alignment is to increase the volume shrinkage rate when drying the base coat.

Water borne base coats reflect much higher shrink rates than solvent borne base coats so brightness can be increased by improving the alignment of the aluminium pigment. However it is necessary to evaporate moisture within the paint that is coated to the body before the clearcoat is applied during pre-heat and this has to be done by force drying.

Audi worked with Dürr, Audi’s general paint shop contractor, to develop a highly automated painting process based on a distributed intelligence and higher-level PC based control system. This enabled processes for new paints and car models to be tested and evaluated.

The process revolves around the supply and removal of air, because variations in air quality and temperature in the paint shop and in baking ovens can greatly impact results achieved for the paint finish.

The system needed to cope with the circulation of air via heaters, heat exchangers and heat emission control systems as well being able to supply and remove water needed for cleansing and removing paint residue in a highly coordinated manner.

Proper control of these various parameters is considered essential so that necessary heat treatments for the aluminium alloy can be performed as well as for optimising the painting processes necessary to achieve acceptable quality levels for the finished paint job.

The flexibility this system provided culminated in Audi being able to apply 15 different series colours and numerous special paint finishes to the body of the A8 despite the problems associated with the painting of aluminium panels.

Painting aluminium, car bodies brings with it a whole new set of difficulties to overcome and as can be seen with some of the other developments in various processes, there has been a need for process innovation to accompany product innovation. In common with many of the other process innovations the adoption of computer technology has played a significant role in presenting the opportunity for much needed increases in the automation levels that seem to be essential for making the developments economically viable.

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