If you are a frequent flier, you are probably very familiar with the flight attendant's pre-flight announcement-how the seat belt works, how to put on your oxygen mask, your seat cushion can be used as a flotation device and check for the nearest exit. In the past several years, the airlines have added another announcement to the end of the pre-flight message-turn off all portable electronic devices until the captain has turned off the fasten seat belt sign.
For someone electronically challenged like myself, it was hard to understand how my CD player, laptop computer or cell phone could have any affect on the airplane. So, why did the airlines add this announcement? The reason is EMI and RFI, or electromagnetic interference and radio frequency interference. When electronic devices are in use, they emit electromagnetic and/or radio waves. When several electronic devices are in close proximity to each other, the waves from one device can hinder the performance of the other. Therefore, your personal electronic devices can interfere with the electronic devices that run the plane.
In order to prevent EMI and RFI, electronic devices are manufactured with a shield. Shielding is a technique used to control the interference between electronic devices by preventing the transmission of electromagnetic and radio waves. Shielding can be accomplished in a variety of ways, but two of the more common methods are plating and painting.
EMI and RFI shielding paints contain metallic flakes, since the coating needs to be conductive. The metallic flakes are usually nickel, copper, silver or some combination of those metals. While using conductive paints is a relatively inexpensive method for EMI and RFI shielding, the paints are still quite expensive-about $125-255/gal-when compared to more common decorative paints.
Shielding Plastic Components
Cybershield of Georgia, Inc. (Canton, GA) is a high-end, high-volume job shop that shields a variety of plastic components for cell phones, radar guns, computers and a variety of other electronics. The company uses a variety of processes to shield these parts, including selective electroless plating, all over electroless plating and conductive painting, but the important process for this article is the conductive painting.
Cybershield has four robotic cells and eight manual booths on a conveyorized line. Before painting, the plastic parts receive a quick anti-static blow-off. In the robotic cells, the parts are then palletized with a specially designed mask placed on top. Each cell has two pallets that shuttle in and out to feed parts to the robot for painting. In the manual booths, parts are fed to the painter by a rack on a conveyor. The rack is then taken off the conveyor by the painter and placed under another specially designed mask. Once the parts are painted, the painter hangs the rack back on the conveyor. Painted parts from either the robotic cells or the manual booths then go through an infrared cure oven and numerous quality checks to ensure that all parts meet the strict demands of Cybershield's customers.
Turbine-Powered HVLP Spray Guns
One of the most important parts of either the robotic cells or the manual booths is the spray gun. Years ago, Jon Pack, plant manager at Cybershield, had the option of using compressed-air or turbine-powered HVLP spray guns. Even though the upfront cost of the turbine-powered HVLP spray guns was significantly higher than the initial cost of the compressed-air HVLP spray guns, Mr. Pack decided to install the Turbo-Coatair turbine-powered HVLP spray guns from Can-Am Engineered Products Inc. Why did he do it? Because the turbine-powered spray guns would greatly increase Cybershield's transfer efficiency and improve the effectiveness of its shields.
With compressed air spray guns, the paint is blasted by high velocity air from numerous small holes in the face of conventional air caps. This creates a large amount of turbulence, which is created by the instantaneous expansion of the high pressure air as it passes through the holes in the air cap. The excessive turbulence and high velocity of the air over-atomize a significant portion of the paint, creating a fine cloud of atomized paint. Because the paint particles are so fine, they ride the air currents in the spray booth, missing the part they are supposed to coat and creating a great deal of overspray. Also, because the velocity of the air, and consequently the paint, some of the larger particles that do strike the part bounce off the part because they are moving so fast.
However, the turbine-powered HVLP spray guns installed by Cybershield use a soft, slow airflow to atomize the coating into relatively uniform droplets and carry the coating to the part. The turbine allows Cybershield to use an air pressure of 3.5-6.0 psi, significantly lower than the air pressure one would see from a compressed-air spray gun.
The soft airflow and relatively uniform droplets provide Cybershield with a number of benefits. The most significant advantage is the increased transfer efficiency. In fact, Cybershield uses such a low pressure that you can't see any paint in the spray booth at all; all you see is paint coating the part. The increased transfer efficiency results in lower paint costs, fewer air emissions when the application requires solvent-borne paint and reduced filter replacement because of the reduced overspray. According to Robert Brewer, engineering supervisor at Cybershield, the increased transfer efficiency saves a considerable amount of money.
Another important advantage for Cybershield is that its air and energy costs are reduced. With compressed air spray guns, the company needed 25 hp to power its four robotic cells. Now, the company uses just one turbine to generate 7.5 hp to power the same four robotic cells. A similar result has occurred with the manual booths where only one turbine is used to power all the manual booths. Plus, the turbines can be turned off when they are not in use. "We also save on energy costs. And, the compressed air requirements are drastically reduced by having the turbine system," stated Mr. Brewer.
One other advantage for Cybershield is the improved finish the turbine-powered HVLP spray guns provide. Recessed areas and corners are easier to coat because the softer airflow of the turbine-powered spray guns doesn't cause the paint to bounce back, which is a common occurrence with compressed-air spray guns. Also, because the paint is so uniformly atomized a more even coating can be applied, which is crucial in the world of EMI and RFI shielding.
Unlike many plant managers, Mr. Pack had the foresight to look beyond the high initial cost of the turbine-powered system to the return on investment Cybershield would get from the reduced paint and application costs. And, if you're thinking that the system is only a benefit to Cybershield because its paint costs are so high, think again. Cybershield is also using the same turbine-powered system for decorative coating of the parts it is currently shielding.