In general, for all applications the preparation treatment for aluminium is as follows:

Clean	Or	Clean
Rinse		Rinse
Etch		Etch
Rinse		Rinse
Chromate		Phosphate
Rinse		Rinse
Demin Rinse		Demin Rinse

Oils and greases are removed in weak alkali or neutral detergent solutions and the surface is etched to remove heavy oxides. After rinsing, the aluminium is dipped into a chromate or phosphate solution to form a conversion coating on the aluminium. This film is chemically attached to the aluminium. After rinsing the aluminium is finally rinsed in demineralised water. Some non-chrome, dried in place pretreatment is beginning to come onto the market; currently, these are not recommended for exterior applications.

The conversion coating has two functions:

It presents a surface to the powder which favours adhesion more than the oxides that form very readily on aluminium surfaces, and

It reduces the incidence of under film corrosion, which may occur at holidays in the coating.

The use of demineralised water reduces the presence of chemical salts on the aluminium surface. These salts have been found to cause filiform corrosion in humid conditions.

For steel the preparation for interior applications may be:

Clean

Rinse

Derust

Rinse

Iron Phosphate

Rinse

Acidulated Rinse

For exterior applications:

Clean

Rinse

Etch

Rinse

Grain Refine

Zinc Phosphate

Rinse

Acidulated Rinse

The grain refiner is used after acid cleaning of steel surfaces and before zinc phosphating, otherwise the zinc phosphate coatings produced will be very coarse with low adhesion. The powder coating applied to a coarse phosphate will produce rough coatings (a little like "sandpaper") and possess low adhesion.

For hot dipped galvanized coatings, which have been stored for more than about 4 hours before powder coating, the following process is necessary for exterior applications.

Clean

Rinse

Etch

Rinse

Grain Refiner

Rinse

Zinc Phosphate

Acidulated Rinse

The etch is required to remove the zinc corrosion products which begin to form almost immediately the zinc is removed from the galvanizing kettle. The grain refiner ensures a fine phosphate is produced.

Powder application processes

The most common way of applying the powder coating to metal objects is to spray the powder using an electrostatic gun, or corona gun. The gun imparts a positive electric charge on the powder, which is then sprayed towards the grounded object by mechanical or compressed air spraying and then accelerated toward the workpiece by the powerful electrostatic charge. There is a wide variety of spray nozzles available for use in electrostatic coating. The type of nozzle used will depend on the shape of the workpiece to be painted and the consistency of the paint. The object is then heated, and the powder melts into a uniform film, and is then cooled to form a hard coating. It is also common to heat the metal first and then spray the powder onto the hot substrate. Preheating can help to achieve a more uniform finish but can also create other problems, such as runs caused by excess powder.

How is it done - electrostatic spray?

The powder is applied with an electrostatic spray gun to a part that is at earth (or ground) potential.

Before the powder is sent to the gun it is fluidised:

to separate the individual grains of powder and so improve the electrostatic charge that can be applied to the powder and

so that the powder flows more easily to the gun.

Because the powder particles are electrostatically charged, the powder wraps around to the back of the part as it passes by towards the air offtake system. By collecting the powder, which passes by the job, and filtering it, the efficiency of the process can be increased to 95% material usage.

The powder will remain attached to the part as long as some of the electrostatic charge remains on the powder. To obtain the final solid, tough, abrasion resistant coating the powder coated items are placed in an oven and heated to temperatures that range from 160 to 210 degrees C (depending on the powder).

Under the influence of heat a thermosetting powder goes through 4 stages to full cure: Melt, Flow, Gel, Cure. The final coating is continuous and will vary from high gloss to flat matt depending on the design of the powder by the supplier.

Powder coating guns

There are at least three types of electrostatic guns in use:

Corona charging guns where electric power is used to generate the electrostatic charge. Corona guns are either internal or external charging.

Tribo charging guns where the electrostatic charge is generated by friction between the powder and the gun barrel. In this case, the powder picks up a positive charge while rubbing along the wall of a Teflon tube inside the barrel of the gun. These charged powder particles then adhere to the grounded substrate. Using a tribo gun requires a different formulation of powder than the more common corona guns. Tribo guns are not subject to some of the problems associated with corona guns, however, such as back ionization and the Faraday cage effect.

"Bell" charging guns where the powder is charged by being "flung" from the perimeter of the "bell"

Not all powder is applied using guns. One system makes use of electrostatic tunnels. Powder can also be applied using specifically adapted electrostatic discs.

Another method of applying powder coating, called the fluidized bed method, is by heating the substrate and then dipping it into an aerated, powder-filled bed. The powder sticks and melts to the hot object. Further heating is usually required to finish curing the coating. This method is generally used when the desired thickness of coating is to exceed 300 micrometres. This is how most dishwasher racks are coated.

Electrostatic fluidized bed coating: Electrostatic fluidized bed application uses the same fluidizing technique and the conventional fluidized bed dip process but with much less powder depth in the bed. An electrostatic charging medium is placed inside the bed so that the powder material becomes charged as the fluidizing air lifts it up. Charged particles of powder move upward and form a cloud of charged powder above the fluid bed. When a grounded part is passed through the charged cloud the particles will be attracted to its surface. The parts are not preheated as they are for the conventional fluidized bed dip process.

Electrostatic magnetic brush (EMB) coating: an innovative coating method for flat materials that applies powder coating with roller technique, enabling relative high speeds and a very accurate layer thickness between 5 and 100 micrometre. The base for this process is conventional copier technology . Currently in use in some high- tech coating applications and very promising for commercial powder coating on flat substrates (steel, aluminium, MDF, paper, board) as well in sheet to sheet and/or roll to roll processes. This process can potentially be integrated in any existing coating line.

Curing

When a thermoset powder is exposed to elevated temperature, it begins to melt, flows out, and then chemically reacts to form a higher molecular weight polymer in a network-like structure. This cure process, called crosslinking, requires a certain temperature for a certain length of time in order to reach full cure and establish the full film properties for which the material was designed. Normally the powders cure at 200°C (390°F) for 10 minutes. The curing schedule could vary according to the manufacturer's specifications.

The application of energy to the product to be cured can be accomplished by convection cure ovens or infrared cure ovens.

How is colour introduced?

Colour is added to powder coatings during the manufacturing process, i.e. before the powder reaches the powder coater. There is little that can be done to change the colour consistently, once the powder leaves the manufacturing plant.

Removing powder coating

Methylene chloride is generally effective at removing powder coating, however most other organic solvents (acetone, thinners, etc.) are completely ineffective. Most recently the suspected human carcinogen methylene chloride is being replaced by benzyl alcohol with great success. Powder coating can also be removed with abrasive blasting. 98% sulfuric acid commercial grade also removes powder coating film. Certain low grade powder coats can be removed with steel wool, though this might be a more labor-intensive process than desired.

Why powder coat?

Powder coating produces a high specification coating which is relatively hard, abrasion resistant (depending on the specification) and tough. Thin powder coatings can be bent but this is not recommended for exterior applications.

The choice of colours and finishes is almost limitless, if you have the time and money to have the powder produced by the powder manufacturer.

Powder coatings can be applied over a wide range of thickness. The new Australian Standard, "AS/NZS 4506 - Thermoset powder coatings", will recommend 25 micron minimum for mild interior applications and up to 60 micron minimum for exterior applications. Care must be exercised when quoting minimum thickness because some powder will not give "coverage" below 60 or even 80 micron. "Coverage" is the ability to cover the colour of the metal with the powder. Some of the white colours require about 75 micron to give full "coverage". One of the orange colours must be applied at 80 micron.

Colour matching is quite acceptable batch to batch.