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OVERVIEW

Corona and tribo charging are two methods widely used in commercial electrostatic powder spray equipment. The most popular method used in North America today is the electrostatic spray process which is comprised of three main components.

Electrostatic Powder Spray Guns
Functions of the electrostatic powder spray gun are to:

  • Shape and direct the flow of powder.
  • Control the pattern size, shape and density of powder.
  • Impart the electrostatic charge to the powder being sprayed.
  • Control the deposition of powder onto the parts being sprayed.


Charging System

The purpose of the charging system is to create a force within the sprayed powder particles enabling them to cling or attach themselves to bare, grounded metal parts as shown below.

 Electrostatic Corona
High voltage or low voltage cables are the two basic ways that the voltage source is currently applied to the tip of a corona-charged powder spray gun. The type of cable depends on whether the high voltage generator is an external or internal power supply gun. Both come in negative or positive polarity. Generally, corona charging uses a negative polarity on the electrode as negative polarity produces more ions and is less prone to arcing than positive polarity. The charging electrode is held at a very high negative potential, requiring a power supply rated from 30,000 to 100,000 volts. Positive polarity guns are used when spraying nylon or touch up in an automatic tribo system. The image below illustrates an external power supply gun and internal power supply gun for voltage generation.

Corona Design Criteria

  • Charge in a region of high field strength.
  • Charge where air velocities are low.
  • Charge where the powder is well dispersed.
  • Arrange the geometry of the charging electrode so the ions flow through the powder stream and the electrode is in the direct line of sight of the ground.

The above design features should be consolidated with the pattern of powder flow, equipment cost(s), ease of maintenance, and durability.

Tribo Charging
The word tribo is derived from the Greek word tribune, meaning to rub or produce friction. In tribo charging, the

powder particles are charged by causing them to rub at a high velocity on a surface and thereby, transferring the charge (Figure 6). Without an external power supply and a charging field in front of the gun, tribo charging virtually eliminates the problem of “faraday cage effect.” The powder particles take on a positive charge inside the gun due to the loss of electrons. The particles are now free to be directed to where they are needed. The powder flows into recessed and difficult-to-reach areas by nozzle direction and air flow. Because the particles are charged in the gun and there are no lines of force, it is less likely that the powder particles will build up on the leading edge of the part being coated. The advantage to this is the near elimination of “fatty edges” resulting in a uniform coating and even film build on the product.

One thing to note, tribo charging is much more dependent on correct powder formulation (chemically) and particle grind size than corona-charging equipment. It is imperative that all powders be tested in tribo equipment to ensure good results.

PF Zone: Gun Design / Nozzle Design

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