Editor's note: This post was originally posted in August 2014 and has been updated for November 2018.
Electrically conductive coatings for plastic enclosure electromagnetic shielding are growing in popularity as concerns over weight increase for a variety of EMI applications in the military, aerospace, automotive, telecom, medical, and semiconductor marketplaces. These paints are filled with conductive metals such as silver, nickel, copper or other specialty powders and flakes.
Conductive paints cannot be applied in the same way conventional paints are. Many first attempts do not go as well as planned. With proper preparation and practice, you will achieve success.
Since conductive paints are infused with heavy metal fillers, metal particles tend to quickly settle to the bottom of the paint. Even the best paint, specifically designed to prevent this settling from happening, will still experience the conductive metals falling out of suspension.
It is critical to avoid this settling while applying conductive paint. To prevent this from occurring, it is suggested to use an air-driven mixer for the paint pot or a recirculation loop that will keep the paint constantly moving. When using a hand-held spray gun, the operator will need to shake the spray cup vigorously between each pass. These methods will help keep your paint homogenous while you work.
You must pay special attention to the paint after it has been stored. Particles will not only settle to the bottom, but also will hard-pack over time. It is important to remember to vigorously shake the container for several minutes to remix the paint. Always verify that the paint is mixed thoroughly by scraping the bottom of the container with a clean spatula or paint stirrer. If there is any sludge of particles found on the stirrer, the paint must continue mixing before use.
Many times on high-volume spray applications, it is less expensive to modify the equipment than to replace the conductive paint if there is a problem. Luckily, due to the significant increase in demand, many manufacturers have modified their own equipment to be compatible with the conductive paints. Currently, there are many hand-held, air-atomized spray guns that are already modified for the application of conductive paints. These guns include the siphon-feed spray gun, the gravity-feed spray gun as well as the high volume, low pressure spray gun with a pressure pot.
Dry spray is an issue that often arises while using conductive paints. A dry spray occurs when the paint does not level correctly, causing particles to not lay down. The goal of conductive paint is to maximize filter particle conduct, which happens by coating the surface with metal filler particles that effectively cover the surface. Dry paint causes the particles to position themselves more perpendicular to the substrate and not fully submerged in the resin. This will cause an extremely rough surface, conductivity loss, and cohesion issues.
Many people believe that if you significantly increase the paint’s coating, the shielding effectiveness also significantly increases, this is incorrect. If everything was done correctly during the painting process, a thicker coat of the conductive paint would add little to no additional shielding.
It is significantly more important to pay attention to uniform thickness while using conductive paint than when using conventional paint. Also, keep in mind issues that may occur with different product designs or inside complicated parts. These problem areas can cause expensive consequences such as failed shield test or even require the redesign of a mold.
After spraying conductive paints for the first time, try not to get too frustrated. Many paint manufacturers are more than willing to work with potential customers to ensure success with their products. With some proper preparation and practice, the entire process will begin to run smoothly and efficiently.
For more information on the causes of dry spray, its solutions or the art of spraying electrically conductive paints, please visit the Chomerics Division.
Original article written by: Jesse Hagar, product development engineer, Chomerics Division, Parker Hannifin Corp.