Powder Coatings 101
Application for Powder Coatings
Today, practically all powder coatings are applied using an electrostatic spraying process. A common factor with all such processes is that the powder particles are electrically charged whilst the object requiring coating remains earthed. The resultant electrostatic attraction being adequate to allow the build-up of sufficient film of powder on the object, thus holding the dry powder in place until melting occurs with subsequent binding to the surface.
Powder particles are electrostatically charged using one of the following techniques:
- Conventional Electrostatic Charging (Corona Charging) by passing the powder through a high-voltage electrostatic field.
- Friction Charging (Tribostatic Charging) which generates an electrostatic charge on the powder as it rubs against an insulator.
Conventional Electrostatic Charging
High voltage (40-100 kV) concentrated at the nozzle of the spray gun causes ionizing of the air passing through the spray gun. Passage of the powder through this ionized air then allows free ions to adhere to a proportion of the powder particles whilst simultaneously applying a negative charge to them.
Between the electrostatic spray gun and the object being coated, the following are present:
It is always of vital importance to achieve the highest possible proportion of charged powder particles during the process itself. The method in which the spraying equipment is used also contributes to success.
Non-charged powder particles do not adhere to the object and will be recycled. Even though recycling is common in powder coating, it is always preferable to keep the amount of recycled powder to a minimum.
Free ions are small and far more mobile than powder particles. Excess free ions will move rapidly towards the object transferring, at the same time, large amounts of negative charge to it. The quantity of free ions being totally dependent on regulating the required voltage. Superfluous high voltage produces an oversupply of free ion, which in turn makes good powder coating harder to achieve and, not least, gives poorer flow (back-ionizing). Insufficient earthing of the object will further worsen the situation.
Using high voltages produces electrical field lines between the nozzle of the spray gun and the object, with the powder showing a tendency to follow these field lines. Objects of a complicated structure will have the highest density of field lines on their outer surfaces, particularly on exterior corners. Similarly, a lower density of field lines will occur on interior corner and indentations.
This phenomenon is commonly referred to as the Faraday Cage effect resulting in difficulties with powder application where the field line density is at its lowest, as shown in the following diagram:
Higher voltage produces a more intense Faraday Cage effect, leading to a thicker film of powder where surfaces are more easily accessible and a correspondingly thinner coating for areas more difficult to reach. It is important to set the spray gun voltage sufficiently high allowing optimum charging of the powder. However using unnecessarily high electrostatic voltage has many undesired effects. What characterizes a skilled powder coating operator is the ability to achieve the right balance.
Powder particles are friction charged as a result of the motion caused by each particle rubbing rapidly against a special type of insulating material which lines the barrel of the spray gun.
Between the friction charging spray gun and the object, as the diagram illustrates, we have present primarily:
With friction charging, there exists no high voltage which can subsequently generate free ions or produce electric fields.
Efficient friction charging of powder particles depends on each particle being rubbed against the barrel of the spray gun. This, as a rule, can be adjusted by regulating the air flow through the gun as well as the powder/air ratio for optimum performance.
Most friction spraying devices are equipped with a microamperemeter that provides an indirect measure of the powder charging process. This electrical current measurement, however, depends on the amount of powder passing through the spray gun. A high mA reading does not guarantee good coating results. The most important factor is maximizing the proportion of charged powder particles existing the spray gun.
Comparison: Two Most Common Techniques for Applying Powder Coatings
Powder Coating by Manual Application
Powder coating is done by a small handgun, ideal for smaller components and mixed goods.
Pre-treatment can be done - as shown- by blast cleaning or any other method suited for the finished product. Curing takes place in a box oven
Automatic Powder Application
Powder coating line with one booth, typical equipment used on large series in one color.
Pre-treatment is either iron or zinc phosphating. The curing takes place in a U-TURN oven. The coating line is equipped with the Jotun Powder Coatings "Big Bag" system.
Powder Coating Line with DIP-Pre-treatment
The coating line is equipped with two moveable spray-booths. Allows easy and fast color changes. Curing is done in tunnel oven. Pre-treatment by dip immersion; chromating and/or phosphating process is usually employed. Typical coating line when coating long items, i.e. extrusions or larger sheets.