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Overcoming the Faraday cage effect in powder application What is the Faraday cage effect in powder coating application, and how is it overcome? M., Wichita, Kans. You know the old legend about the New Yorker who stops Arthur Rubenstein on the street and asks: "Pardon me sir, but how do I get to Carnegie Hall?" Ruben- stein replies, "Practice, practice, prac- tice." Well, the same applies to Faraday cage effects when powder coating. Op- erator technique and application gun settings (both are perfected by experi- ence) are the best way to overcome this issue; the more you coat parts with this problem, the better you get at it. The Faraday cage effect is caused when a part has a recess or a protrusion that creates a tight inside corner on the part surface. The electrostatic field lines of an application gun tend to be attracted to the closer surfaces (flat areas) and to avoid the corners. Powder charged by the electrostatic field at the gun tip fol- lows the field lines, easily coating the flat surfaces. As charged powder particles build onto the flat surfaces, driving powder into the recessed corner be- comes even more difficult because the powder particles on the flat surface have the same polarity as the particles being sprayed to coat the recessed area. We all know from high school science that like-charged particles repel and oppo- site-charged particles attract. What you're left with is a heavily coated flat surface with an uncoated recessed cor- ner. This is called a Faraday cage area. Faraday cage areas have been around as long as electrostatic powder coating has been practiced. The first issues that must be addressed are the operational parameters where the powder coating takes place. For instance, the environ- ment must be humidity-controlled be- cause low humidity (below 40 percent relative humidity) will increase the Faraday effect. The part must be prop- erly grounded, since ungrounded parts have more severe Faraday issues. The gun voltage must be adjusted to reduce the charge on the powder particle be- cause highly charged particles are more difficult to drive into the recessed areas and are more easily attracted to the flat surfaces. Some guns have internal charging con- trols or features that should be em- ployed to overcome Faraday areas. Gun aerodynamics have a dramatic influence on how well a Faraday area is coated be- cause you can easily displace the powder in the corner with high-velocity air coming from the gun nozzle. Selecting a different nozzle for the gun to slow the powder velocity can be very helpful in coating recessed areas. If all else fails, a tribo-charging (frictional charging) gun can be used to coat the most difficult Faraday areas, as they don't have elec- trostatic field lines that cause the Fara- day problem in the first place. Finally, operator coating technique is very important when spraying Faraday areas. The well-trained operator will first coat the recessed corner before coating the adjacent flat areas to minimize the rejection of like-charged particles. That operator will also remember to select the required gun voltage and gun tip, and slow the powder velocity to ensure proper coating of the Faraday area. Hav- ing all the right tools and proper condi- tions mean nothing if the person spray- ing the part doesn't know what they're doing. Remember the old adages: Prac- tice makes perfect, and there is no substi- tute for experience. —N.L. Requiring specifications from customers Is there a standard spec out i n t h e p o w d e r c o a t i n g world related to appear- ance? We have a client who has a spec for "appearance in a certain light 3 feet from the product." Is there a standard for those who don't have a spec? M.C., Monroe, N.C. The end user or customer must have a specification that calls for a certain appearance on the coating. If the customer can't de- fine this, you legally have an open door for appearance requirements. A cus- tomer would be a fool to do this, but I know I'm not the only fool in this world. There are small light booths available that have several different lights mounted in them to duplicate lighting such as artificial light and natural sun- light. Your coated part should match color chips (supplied by your customer or the customer's powder coatings sup- p l i e r ) i n b o t h l i g h t s , u n l e s s i t ' s metameric by design. Your customer may have a requirement for distinctiveness of image (DOI), which in part deals with flatness versus orange peel of the finished coated sur- POWDER COATING, November 2016 25 Questions & Answers George Trigg GRT Engineering Nick Liberto Powder Coating Consultants Q A Q A