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THE APPLICATION OF POWDER COATING

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THE APPLICATION OF POWDER COATING

The most common method of powder coating among listed in “Fundamentals of powder coating” is pneumonic-electrostatic spraying.

Today it is the main method of coating in industries. High performance, simplicity of design, ease in use, spraying guns, and an opportunity to control thickness of layer contribute to its popularity.

The pneumonic-electrostatic method of spraying is quite simple. You should charge particles with the help of mechanics and electrostatic laws, and then transport and evenly distribute them to the surface. You should keep them in such conditions until placing it into curing oven.

The simplest diagram of powder coating process is presented below:

  1. Compressor.
  2. Control panel.
  3. Receiver.
  4. Air cleaning unit.
  5. Pressure regulator.
  6. Powder supply unit.
  7. Recovery system.
  8. Spraying gun.
  9. Powder charging unit.

Blocks (1-7) are typical and they are used in many processes. Only powder charging units and spraying guns are specifically used in powder coating industry.

Charging of powder particles is mainly produced in two ways:

  • electrostatic;
  • tribostatic.

Charging mechanisms in each method are completely different. They impose several specific requirements as to characteristic of spraying materials, so to spraying conditions. Currently, large theoretical and experimental database is accumulated in order to evaluate all advantages and disadvantages of each method.

However, charging by corona discharge is more often tribostatic. According to practice, it is more effective, because it has higher degree of electrification for majority of powder paints. In our work we use both electrostatic and tribostatic spraying guns of our own development (see photo and section “Equipment for powder coating”). But preference is given to tribostatic spraying gun.

It is known that efficiency of tribo charging is determined by the following factors:

  • the ratio of charged surface to powder volume which depends on trajectory of particles and degree of air-powder mixture turbulence;
  • distribution of powder particles by size;
  • optimal selection of charging surface material.

Basically, type of modern tribostatic spraying guns is called «acceptor», because they use materials which can take charge from the particle.

The most appropriate material which is used as charging surface inside of tribostatic spraying gun is fluoroplastic. (see «tribostatic line»).

Control of particle trajectories in such systems is generally limited by construction of charging pipe. In our opinion this method doesn’t study opportunity to control trajectory of charged powder by addition of controlled electrostatic and magnetic fields. For example, if construction of charging system allows such air-powder mixture movement in which every particle acquires a radial component of velocity, so longitudinal magnetic field will cause particles to move along helical trajectory (Lorentz force). As a result effective contact inside spraying gun increases and thereby the rare of tribocharge also increases.

So the task to develop ‘donor’ tribostatic spraying guns, where charging surface donates electrons has become more topical. It connects the widespread use of thermoplastic paints. Our experiments in tribo-charging of thermoplastic powders allowed to reveal some features and practical recommendations. In particular, methods of trajectory control were defined and optimal conditions of powder applying were selected (see photo).

As the average size of these particles (median) is in the range of 250 microns, it was necessary to reconsider the question of particle’s transportation. We had to take into account changes in interaction of electric hydrodynamic, inertial, gravity forces which depend on particle’s size, weight, dielectric constant and flow density.

In conclusion it should be mentioned that main requirement for all chargers is the stability of parameters in applying process.

The stability criterion can be constant specific particle’s charge, which is approximately 0.5-2.5.

It is quite important to say that issues of powder coating applying which are linked with overall problems of powder coating are not fully studied.