Powder Coating Faraday Areas

Contributed by Jeff Wilhelms

Achieving adequate powder coverage in Faraday areas has been a challenge for powder coating operations for many years. To effectively address this issue, it is important to first understand the Faraday cage effect.

The Faraday cage effect occurs when electrostatic field lines 

concentrate on the outer edges and surfaces of a part, making it difficult for charged powder particles to penetrate deep recesses, corners, channels and other enclosed geometries. As a result, powder tends to build on external edges while recessed areas receive insufficient film coverage, potentially leaving those areas vulnerable to future corrosion.

One of the most effective tools for improving coverage in these difficult-to-coat areas is current limiting, often referred to as AFC (automatic feedback current). AFC automatically adjusts the gun's electrostatic output based on the amount of current being drawn by the grounded part.

Unlike operating at a fixed KV setting, AFC reduces excessive charge buildup on edges and corners, enabling powder particles to better penetrate recessed areas. This results in more uniform coverage, improved transfer efficiency, reduced edge buildup and fewer defects such as back-ionization that can negatively impact appearance.

While AFC is an effective solution, proper application technique remains critical. For best results, parts must be properly grounded. Once a good ground connection has been established, operators should:

1.      Spray recessed areas first.

2.      Reduce powder flow and/or air settings when necessary.

3.      Maintain an appropriate gun-to-part distance.

4.      Apply multiple light passes rather than one heavy coat.

Following these best practices can significantly improve film uniformity and overall coating quality across complex part geometries.

Preheating the part before powder application is another technique that can enhance coverage in Faraday areas. When powder particles contact a warm surface, thermal attractions occur, and they begin to melt and flow more readily into deep recesses, corners and channels that are difficult to coat through electrostatic attraction alone.

However, preheating must be used carefully. If the part temperature is too high, the powder may begin curing prematurely, potentially resulting in appearance or performance issues. Elevated part temperatures can also cause film thickness to build rapidly, increasing the risk of extremely heavy powder deposition. Consistent part temperatures are essential to achieving uniform coating results.  Although preheating can be beneficial, it is generally considered a secondary technique.

Proper grounding, optimized AFC settings, reduced powder/air outputs, spraying recessed areas first, or applying multiple light passes remain the most effective methods for achieving consistent coverage in Faraday areas and ensuring long-term coating performance.

Jeff Wilhelms is the Field Technical Service Manager at TCI Powder Coatings.

Applying powder to corners effectively can be achieved by implementing some best practices. Photo by Steve Ladatto, courtesy of Gordon Inc.