The Faraday Cage Effect and 120kV Powder Coating Technology in Indonesia

The problem: heavy build-up outside, thin coverage in corners

If you have seen powder coating results on switchgear panels, racks, or hollow profiles, the pattern is usually the same: outer surfaces are covered neatly, while internal corners and cavity interiors look thin or still show bare metal. Spraying for longer usually does not fix it — the outer surface becomes too thick while the internal corners stay short.

This is not an operator being careless. The cause lies in how charged powder travels toward the part, and it is known as the Faraday cage effect. Understanding the mechanism matters because it determines what can genuinely be fixed by settings and what has to be solved by equipment capability.

This article covers the technical side only. For what an automatic powder coating line contains and what each piece of equipment does, there is a separate article on Electron line equipment.

What the Faraday cage effect is

In electrostatic powder coating, powder particles are charged as they leave the gun and are then drawn toward the grounded part. What pulls them is the electric field between the gun tip and the part surface.

That field is not evenly distributed on a complex part. It is strongest at the most prominent features — edges, external corners, surfaces facing the gun directly. In internal corners and inside cavities the field is far weaker, because the surrounding surfaces shield one another.

As a result, charged particles are drawn to edges and outer surfaces first. Once powder builds up there, that layer itself begins repelling further particles carrying the same charge. Meanwhile the weak field in internal corners is not strong enough to pull powder in. That is the Faraday cage effect: the part's geometry forms a cage that keeps the electric field out of shielded areas.

• The electric field is strongest at edges and external corners
• Internal corners and cavities receive a far weaker field
• Powder already deposited repels further particles of the same charge
• Spraying longer worsens outer thickness without improving the interior

Which parts are most affected

Not every part is a problem. Flat parts such as sheets or simple panels are usually fine on standard settings. The difficult ones are parts whose geometry creates shielded areas:

If your production is dominated by parts like these, how well the equipment handles the Faraday cage effect bears directly on reject rates and on how many parts need reworking.

• Switchgear panels and enclosures — many internal corners, flanges, and opposing faces
• Wheels and disc-shaped parts with gaps — the area between spokes is hard to reach
• Hollow extruded profiles — the interior receives almost no field
• Racks, baskets, and mesh-form parts — surfaces shield one another
• Parts with deep holes or grooves — depth exceeding the opening width

Why higher voltage helps

Higher gun voltage produces a stronger electric field. A stronger field means the attractive force is still meaningful in areas where the field is weak — precisely the internal corners and cavities that were out of reach. This is why guns operating at 120kV penetrate better than lower-voltage guns on the same part.

But voltage alone is not the whole answer, and this is where the common misunderstanding sits. Very high voltage without good current control can create a new problem: excessive air ionisation around the gun tip produces free ions that also land on the part surface. Those free ions build an opposing charge in the deposited powder layer and can trigger back-ionisation — powder already deposited comes loose, or the surface develops a pitted appearance.

So what determines the result is not the peak voltage figure but the combination of voltage and current setting. Current control limits free ions so penetration improves without triggering back-ionisation. For complex parts, high voltage with limited current usually gives a better result than maximum voltage with no limit.

The effect on transfer efficiency

Transfer efficiency is the ratio of powder that actually adheres to the part against the total sprayed. The Faraday cage effect lowers it from both directions at once: powder that fails to reach internal corners is lost to the booth, and excessive build-up on outer surfaces means more material is used than needed.

Improving penetration therefore pays twice. Parts finish in a single pass without rework, and more of the powder sprayed lands where it should. At high production volumes that difference shows in monthly powder purchasing.

How large the improvement is differs by plant, because it depends on part geometry, powder type, gun distance, and the settings in use. We do not quote a general percentage here — the honest way to find out is to run your own parts. That kind of trial can be done at our demo lab at MM2100 Cibitung.

What to check before changing equipment

Before concluding that equipment needs replacing, a few things are worth checking because they are frequently the real cause:

If internal corners are still short after all of this, then gun voltage capability and current control become the deciding factor. For a technical discussion about your specific parts, our engineering team can help work through it.

• Part grounding and hanger cleanliness — poor grounding weakens the whole electrostatic mechanism
• Gun-to-part distance — too close worsens build-up on outer surfaces
• Spray angle — aiming into shielded areas is often more effective than spraying longer
• Current setting, not only voltage — excessive current triggers back-ionisation
• Powder condition — damp or aged powder flows and charges inconsistently