What is Back-Ionization?

Q. We spray parts using a silver metallic powder by hand. The surface has a lot of texture and small stars in certain areas and our powder supplier says it is “back-ionization.” What is back-ionization and how do we overcome the problem?

A. Let’s start with corona charging. The way a corona gun charges is by discharging high voltage from an electrode at the gun tip. The voltage may have the potential to reach 100 kV while the micro-amps remain under 100. The high voltage collides with air molecules in the atmosphere and splits them into a negative component (electrons or ions) and a positive component (proton). The ions collect on the powder or flow to the grounded part. The proton is attracted back to the negative electrode. The charge from the free ions attracts the powder to to the part and sticks to the surface.

Voltage and amperage can change based on resistance between the gun electrode and the part. Move the gun closer and resistance goes down. This creates a higher current flow (9 microamps) to the part and reduction in voltage. Move the gun farther away and the opposite occurs; current flow is reduced due to greater resistance and voltage goes up. 

Back-ionization is caused by a radical jump on current at the part surface. If the gun gets so close that the current jumps up, it can cause some problems with orientation of the powder particles at the surface. In some cases, the breakdown of air molecules can take place under the film because of the extreme amount of energy present from the radical jump in current. The proton is pulled back to the gun electrode and it breaks up the film as it goes, causing a rough, textured film in that area. This is back-ionization; too much current (micro-amperage) in a small space.

You can cut down on voltage, which reduces the current but also may reduce charging efficiency. A better approach is to reduce micro-amps. Most newer gun technology has a current limiting adjustment so you can reduce to a maximum level. Experiment with different current levels to see if you can find the best level for that powder and your parts. 

If too close to the part, you can cause back-ionization even at a lower current level. Make sure operators are trained and understand gun-to-target distance. A manual gun can be moved in to reach inside corners but not too close and keep micro-amps lower when tight to the part surface. 

Orange peel can be caused by excessive film thickness. While this is true to a certain extent, a contributing factor is electrostatic behavior. The film starts to increase resistance by “insulating” the part surface. This causes a drop off in film development with an increase in larger particle deposition and a reduction in smaller particle size deposition. The increase in resistance can generate more rapid back-ionization. So be aware of the role that film thickness has on the back-ionization issue and avoid too much thickness.  

 

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Originally published in the June 2017 issue.