October 2005 Archives

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Powder Coating

Pre Treatment of Components Prior to Powder Coating

It cannot be emphasised enough how important is the pre-treatment of surfaces prior to coating. The failure to ensure correct cleanliness prior to powder coating can lead to at best a poor finish and at worse the premature failure of the surface, leading to costly repairs.

One of the most important features to get right is the correct balance of cost over performance - all too often components can be over-specified, leading to expensive and uncompetitive processing costs, but more often than not under specification leading to premature failure.

There are several processes for pre-treatment prior to powder coating, but the following three are those mainly used - each with its own merits:

1. Degreasing. This can vary from a straightforward wipe with a rag soaked in a proprietary degreasing liquid, to quite complex ultrasonic vapour cleaning. The contaminated surface will be clean and will dry fairly quickly to be ready for painting almost immediately, depending upon the evaporation rate of the solvent. This method is a mixed blessing as the environmental issues can be a problem if not correctly administered. It is, however, the system to choose if cost and simplicity are important considerations.

2. Phosphate conversion coatings. This both cleans and etches the material, providing a crystalline surface. There are two basic types: zinc and iron, both offering excellent durability, with the zinc phosphate coming out better for performance, and the iron better for its more sympathetic environmental consideration. This latter feature makes this system best for the balance between performance and cost.

3. Galvanising. This is a loose term that most people associate with the spangle silver finish on fences and gates, but in reality it covers all metal surfaces covered with another metal - usually in order to prevent corrosion. Thus chromium-plated objects, are strictly speaking, galvanised.

More often than not the galvanised surface is zinc based. Because this is seen as a finish in its own right, further coating can be problematic if not treated correctly; this system is the preferred one when performance is the main consideration.