Procedures of Powder Coating

Procedures of Powder Coating

The powder coating process consists of three steps:
  1. Part preparation or the pre-treatment
  2. the powder application
  3. curing
In part preparation, there is removal of oil, metal oxides, welding scales, soil that is necessary to be done before the powder coating process. There are several chemical and mechanical means of doing this.
In powder application process, the usual method of applying the powder coating to metal objects is spraying the powder by using an electrostatic gun or Corona gun. This gun provides a negative electric charge on the powder. This is then sprayed towards the object which is grounded.

This object is then heated. The powder melts in a uniform film and then cooled to form a hard coating. It is a common method to heat the metal initially and spray the powder on the hot substrate. Preheating enables to have a more uniform finish. However, there may be other problems like runs due to excess powder. Another type of gun is called Tribo gun. This charges the powder by triboelectric function.

The powder picks up the positive charge while rubbing along the wall of a Teflon tube in the barrel of the gun. These charged powder particles stick to the grounded substrate. Using a Tribo gun requires a different mixture of powder, than the more common Corona guns. Tribo guns are not concerned with some of the problems linked with Corona guns. Some of these problems are back ionization and the Faraday Cage effect. Specifically adapted electrostatic discs are used for applying powder.

A different method of applying powder coating is called Fluidized Bed. In this, the substrate is heated and dipped in an aerated, powder-filled bed. The powder melts and sticks to the hot object. More heating is required to finish the curing of the coating. This method is usually used when the desired thickness of coating becomes more than 300 mm. This is the usual method of coating most dishwasher racks.

In Electrostatic Fluidized Bed Coating there is less powder depth in the bed. An electrostatic charging media is kept in the bed. As the fluidizing air lifts it up, the powder material becomes charged. Charged particles of powder move upward and a cloud of charged powder above the fluid bed. When a grounded part passes through the charged cloud, the particles will be attracted to its surface. The parts are not preheated.

In Curing, when a thermoset powder is exposed to elevated temperature, it melts, flows out and then chemically reacts to form a higher molecular weight polymer in a network-like structure. This cure process called crosslinking needs a certain degree of temperature for a particular length of time in order to reach full cure and establish the full film properties for which the material was designed.

The powders cure at 200 degrees celsius in 10 minutes. Convection cure ovens or infrared cure ovens are used to serve the application of energy to the product to be cured.




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