Application of Powder Coatings
Before a powder coating can be applied it must be converted into a fluid otherwise it will not pass through the application system and will not result in an evenly applied coating. This is done by passing dry, clean air through the powder, via a semi-permeable membrane, in a hopper or box feeder system. There are 2 main methods which can be used to apply the powder to an object once it has been fluidized.
- Spray application
- Fluidized bed dipping
In spray application, the powder must be transported from the fluid bed or box via tubes to the spray guns. Once the powder is fluidized as described above it can be transferred using venturi or dense phase pumps – more information about these can be obtained from you application equipment manufacturers.
The concentration of the air/powder mix can be adjusted to control the overall application performance and is one of the parameters used to control the film thickness of the coating.
The application of powder to the workpiece uses the principal of attraction between objects with different charges. The charged air and powder particles from the gun are attracted to an earthed substrate – the object to be coated.
The charging process is critical to the powder coating application processes, if the powder is not charged effectively it will not adhere to the surface of the metal.
The 2 main methods for spray application of powder coatings use different approaches to charge the powder are:
The most common application method is electrostatic spray, often called “Corona” application.
In this method, a cascade is used to generate a voltage - up to 100kV - which is passed through an electrode at the tip of the powder gun. The high voltage at the gun tip results in a corona, the air is ionized and an electric field is created between the electrode and the earthed object. Normally, the corona is negatively charged resulting in a flow of negative ions from the gun to the substrate. Powder particles pick up a negative charge from the corona and ionized air molecules and flow to the earthed substrate where they are held in place by the electrostatic attraction.
Film thickness is controlled by a combination of electric field strength, gravity, air pressure and powder concentration.
There are many types and shapes of Corona spray guns including electrostatic bells and disks.
In this method, powder particles are charged by friction when they pass through a PTFE tube. The powder particles lose electrons during this process and become positively charged. The charged powder particles move with the air flow towards the earthed object where the charged powder cloud creates an electric field and an electrical attraction between the powder and the object.
The extent of frictional charging is governed by the volume of powder delivered onto the charging surface and the speed it moves across the surface. Both of these parameters can be controlled by the gun operator to achieve the desired coating thickness and finish.
Generally, air flows and electrical field strengths between the gun and the substrate are lower with tribostatic in comparison to electrostatic so they perform better when trying to coat recesses, or Faraday cages, in objects.
Fluidised Bed Dipping
In the fluidized bed application method, the object is pre-heated to a temperature above the melting point (Tg) of the powder. The object is then submerged in, or in the cloud just above, the fluidized bed of powder, the Powder melts and forms a coating onto the hot surface.
When the object is removed from the fluid bed it normally passes through another oven to fully cure the powder. The film thickness is controlled by the initial temperature of the object when it enters the fluid bed and the time it spends in the fluidized bed above the Tg of the powder.