TCI Application & Troubleshooting Guide

Chapter Nine: The Electrostatic Charge Process

Corona Charging
  • Typical voltages of 40,000 to 100,000 VDC.
  • Typical currents of 15-60 Micro amps
  • One milliamp is 1/1,000th of an amp and would cause a small but noticable shock
  • One micro amp is 1/1,000,000th of an amp and would cause no sensation at all.
  • 15-60 micro amps will not harm humans though extended exposure might cause tickling sensation-similar to a carpet shock.
  • Symbol for Micro amp is: µA

Corona Charged Electric Field Lines
  • The corona field creates field lines of force coming out of the gun which converge on sharp edges and points.
  • Free electrons follow the lines of force.
  • The electrons hit air molecules splitting them into 2 more electrons and 1 ion.
  • New electrons hit new air molecules
  • Free ions travel towards the source.
  • Ions also hit and split air molecules.

Corona charging electrostatic spray powder coating application methods - Corona charged electric field lines

     
Corona Charging of Powder Particles
Corona charging of powder particles
  • The uncharged powder particle will attract field lines.
  • Free ions will begin to be captured by the particle.
  • The particle continues absorption until it has the same potential as the incoming ions
  • Degree of charge absorbed depends on particle size, field strength, and time in charge area
 
Corona Charging Particle Saturation
Corona charging of powder particles saturation
  • When the particle has reached its saturation point of captured ions it develops its own electric field.
  • This new field will then cause lines of force to be pushed away from the particle.
  • Ions can no longer reach the particle due to repulsion.

Several forces act on the moving particle to deliver and deposit it on the part: air resistance, aerodynamic force, electric force, and gravity. Importantly electric force can only complete its function if the product substrate is properly grounded.

Electrostatic Attraction
     
Electrostatic Attraction of Powder at the Substrate Surface:
  • Powder will retain a charge for several hours (minimally) if grounded properly.
  • As a powder contacts a grounded surface it induces an equal and opposite charge on the surface of the substrate.
  • This occurs because like ions are repelled from the area.
  • This reaction is called a "mirror" charge and serves to hold the powder particle in place.
  • The larger a particle (retained charge) the stronger the charge and attraction.

Electric Deposition of the Powder:
  • Large particles typically accumulate stronger charges. Therefore larger particles will tend to build on top of smaller particles deposited more directly on the surface.

Back Ionization:
Back ionization is caused primarily when the part has reached the saturation point at which no additional powder can be attracted to the substrate. Indications may be:
  • Limited film build
  • Powder attracted to the applicator, not the part
  • Star/swirl patterns

As powder continues to be deposited on the part, the strength of the electric field within the powder layers increases because each new particle:
  • Increases cumulative charge of the film and
  • Increases the cumulative mirror image.

If spraying continues beyond the saturation point, the electric field within the film will become high enough to:
  • Inhibit deposits of additional particles
  • Ionize trapped air within deposited particles
  • Split more air molecules with new +/- ions
  • Cause electron streams to rush through the coating towards the ground repelling charged particles
  • Cause ion streams to rush through the coating toward the gun, the canceling charge of particles
  • Cause these streams to disrupt deposited powder film ripping powder away and creating voids and craters, or star patterns, in their wake.

Adjustments for back ionization include:
  • Checking the ground and hook cleanliness
  • Increasing gun to part distance
  • Decrease voltage by 10-20Kv
  • Insure reclaim to virgin mix is correct
 
Electrostatic methods of applying powder coatings - Electrostatic attraction diagram

Particle deposition of powder coating particles - powder coating troubleshooting guide

Back ionization of powder coating particles - powder coating troubleshooting guide



Conventional Electrostatic Units
     
Current Output
Current output and gun to part distance for applying powder coatings - powder coating troubleshooting guide
 
Transfer Efficiency vs. Gun to Part Distance
Gun current compared to transfer efficiency for applying powder coatings - powder coating troubleshooting guide
Faraday Cage Effect Basics

Attractive forces are inversely proportional to the square of the target distance.

FORCE =
I
---------------
(DISTANCE)²



Current Output

Current output for applying powder coatings - powder coating troubleshooting guide

A to B = 2" (1/(2x2) = 0.25 or relative force =16

A to C = 4" (1/(4x4) = 0.0625 or relative force = 4

A to D = 8" (1/(8x8) = 0.0156 or relative force = 1



Contributing Factors
  • Edges build quickly, and can back-ionize before recesses and other areas can coat as in the figure at right
  • Corona charging creates strong electric fields
  • Edges offer the least amount of resistivity
  • Electric fields follow paths of least resistance
  • Charged particles follow with lines of force
  • The less the distance the more the attraction
 
Electrostatic powder coating difficulties - recessed edges
  • When coating recesses, the amount of powder deposited is also limited by the extra surface area to be covered
 
Electrostatic powder coating difficulties - recessed edges
  • A: The Space Charge
    Consists of charged particles and free ions which also creates its own electric field toward the part.
  • B: Lines of Force
    Corona generated charge field and lines of force from the electrode.
 
Electrostatic powder coating difficulties - recessed edges
Effect of Particle Size on Surface Area and Charge

The charge developed by a powder particle is a function of the charge to mass ratio. This ratio is inversely proportional to the radius of a particle. Larger particles charge less efficently. A (one) 1 mil particle has 1/512th the mass of an 8 mil particle, but the total volume of smaller particles making up that mass will carry 8 times the charge due to the increased surface area.

Base Size Number of Particles Surface Area
8 Mils 1 Particle 1 Unit
4 Mils 8 Particles 2 Units
2 Mils 64 Particles 4 Units
1 Mil 512 Particles 8 Units
Current Limiting Devices

Current limiting devices were developed to help minimize back ionization and are particularly suited for automatic guns where constant adjustments are impractical. As the air space between the gun and part decreases, the resistance also decreases, therefore the current increases proportionately. With a higher current, more and faster free-ions are generated. Higher current causes the part to reach saturation and back-ionization making it more difficult to build film and coat recess. These devices assist in reducing Faraday cage effects and help maintain optimum field strength and gun current between the electrode and parts.

Pauthenier’s Equation

The ability of powder particles to develop a charge while passing through a corona field is governed by Pauthenier’s equation.

Pauthenier’s equation: governs the ability of powder particles to develop a charge while passing through a corona field



WHERE
r = Radius of particle
E = Field strength
e = Charge of an electron
k = Electron mobility
n = Electron concentration
t = Time
?? = Absoulute permittivity
?r = Relative permittivity of powder material

The amount of charge is directly proportional to the field strength geometry and the square of the particle radius. It is also affected by the particle and the amount of time in the charge zone.

 

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