The oldest method of powder application is the Fluidised Bed process.
In this process, a preheated substrate is immersed into a fluidised powder bed. The action of heat and the powder coming into contact with the substrate results in the melting of the powder and its adherence to the substrate.
The fluidised bed is constructed as a two-compartment container with the top open for dipping substrates to be coated into the powder fluidised within the bed. The fluidised bed utilises an upper compartment for storage and fluidisation of powder; a lower compartment is provided as air space where the compressed air is regulated through a porous membrane separating the lower and upper compartment.
· The thickness of a coating is determined by the temperature of the preheated substrate and the total immersion time
· Generally a coating of 150 microns – 300 microns is obtained
· The compressed air must be clean, dry and oil-free, and regulated to achieve an optimum level of fluidisation
Electrostatic spraying is the most widely used method of applying powder coating.
In an electrostatic powder spray system, powder is supplied to the spray gun from a feeder unit where the powder is fluidised. The powder is pumped from the feeder unit through powder feed hose to the spray gun. The spray gun directs the powder towards the substrate to be coated in a diffused cloud.
The air used to transport the powder from the feeder unit provides the propelling force.
An electrostatic voltage is applied to the spray gun by means of high voltage generation, low amperage electrical powder to an electrode attached to the spray gun. As the powder particulars flow through the gun, it picks up a charge from the electrostatic field near the electrode. The electrostatically charged powder particles will be drawn to the grounded part by electrostatic attraction force.
The overspray powder can be collected, filtered and recycled to the feeder unit, either automatically or manually. The recovery unit usually consists of a cyclone and filter bags.
This method of powder coating is the latest invention up to now. It has not gained as much acceptance as the conventional Electrostatic Powder Spray system. Unlike the electrostatic spray gun, the tribo-matic gun does not have an electrode. The charging of the powder particles is achieved by friction-rubbing between the particles and the transfer tube. At present, most of the tubes are made of Teflon PTFE. It induces a positive charge to the powder particle.
With the absence of strong electrostatic field, the charged particles are able to penetrate into the deep recesses of complicated-shaped objects, overcoming the ‘Faraday Cage’ effect.
This system also produces a more even coating with superior finishes – less pin-holes and craters due to the ‘back-ionisation’ effect.
The main set-back of this method of application is the poorer charging capacities of each gun compared to the conventional electrostatic gun – i.e. more tribo guns are needed for the same through-put of total area coated.
The charging ability also depends on the type of resin system used.