POWDER COATING

Proper surface preparation is the single most important aspect of a powder finishing system should you wish to maximise powder performance. The following tests and requirements are directly influenced by the type and quality of the pretreatment on the metal substrate that is to be powder coated:

• Dry-film adhesion

• Cross Hatch Adhesion

• Impact Testing

• Zero T Bends

• Post Forming

• Accelerated Humidity Testing

• Salt Spray Testing

The procedure required to obtain a suitably prepared surface depend heavily on the type of substrate but in most cases the following steps are required:

Cleaning

The cleaner will depend on both the substrate and the soils to be removed e.g. synthetic draw lubricants are much easier to remove than a buffing or polishing compound. Metal oxides (rust) need to be removed at this point as they will act as a barrier to powder coating film adhesion. This is normally achieved by using an acid deruster

The effectiveness of the cleaning operation can be checked by:

Water Break Test - Pure uncontaminated water is poured over the surface. A uniform sheet of water (no beading) will indicate that the surface is free of organic soils.

White Towel Test - Wiping a white paper towel across the dry surface will indicate the effectiveness of inorganic soil removal 

Conversion Coating

Conversion coatings are materials and processes that impart corrosion resistance better adhesion properties to a metal substrate. The type of conversion coating depends heavily on the nature of the substrate as well as the desired properties of the coating.

Aluminium - The aluminium surface is treated with a chromate conversion coating to produce a surface layer that comprises of both aluminium and chrome oxides. This is a smooth uniform surface that is chemically inert and allows good adhesion of the organic coating. A chrome-free conversion coating is available for environmental reasons. 

Iron & Steel - Zinc Phosphate or Iron Phosphate is used depending on the mechanical properties and corrosion resistance required from the finished part. Various products are available, including the cleaner / coater option which reduces the number of stages and consequently the equipment required.   

Zinc Alloys & Galvanised Surfaces - Good adhesion is difficult to obtain on a zinc surface and this problem is increased by spangle. A Zinc Phosphate coating is required for good powder adhesion, Bonderite 34BR allows a "wipe-on" solution for low volume production. 

Passivation

A passivator (seal rinse) is applied over a zinc or iron phosphate to increase the corrosion resistance of the coating and to improve adhesion of paint and organic finishes. Passivation is important if components are going to be stored or transported between chemical treatment and powder coating. 

Rinsing

The work pieces are rinsed between stages to prevent cross contamination of process solutions and to reduce chemical consumption. The final rinse prior to drying is to remove any unreacted chemical or salts that could adversely affect adhesion. Remember that a part is no cleaner than the quality of the rinse water used.

Click here for a process example - Mild Steel Components

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