Metal Finishing Guide Book


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manual powder coating operations. The greater repeatability that is achieved through robotics—when coupled with dense phase powder coating delivery and application equipment—delivers even greater levels of productivity and more savings than would otherwise be achieved with conventional methods of moving guns (fixed axis or manual operator) and venturi pump powder delivery and application equipment. Dense phase technology and robotics—for greater application control and efficiency. So, what is dense phase technology? It is essentially dense phase powder pumps, guns and controls that are superior to venturi powder coating equipment and all other dense phase powder equipment on the market. Previously, automatic powder guns with conventional venturi pumps could not paint fast enough, or effectively enough, to take full advantage of robotic technology. However, the advent of dense phase powder pumps and spray guns has created a perfect match for the speed, accuracy and repeatability of today's painting robots. Dense phase technology was born of the need for 20-second color changes, hundreds of colors on a powder coater's color palette, and same day shipments– therefore, the need for many color changes not only within a single 8-hour shift, but within a single hour. Dense phase technology was initially developed for North American office furniture manufacturers that were spraying hundreds of different colors and needed to ship complete office sets (desk, chair, file, drawers, etc.) the same day they received an order. They eventually determined that it was less costly to throw powder away than to have any amount of significant downtime for color changes. These same customers were also running line speeds as fast as their conventional powder equipment would allow them—in order to coat as much product as possible. This was initially achieved by manually switching air and powder lines between multiple hoppers with different colors, equipped with venturi pumps and manual powder spray guns. However, venturi pumps did not lend themselves well to powder coating at high line speeds because so much air was required to propel the powder—propelling much of it past the part when the air flows were turned up to achieve greater powder flow in order to accommodate higher line speeds. It was here that a "dense phase" delivery pump was developed (Figure 1). This pump was designed to deliver more powder at lower velocities because it was more of a metering pump with two chambers. The first chamber with two valves would pull the powder in with vacuum and push the powder out with pressure. The second chamber would operate similarly but its timing would be opposite that of the first chamber, such that when the second chamber applied pressue, for example, to push powder out—the first chamber would apply vacuum to pull powder in. Alternating cycles resulted in smooth dense powder flow with no pulsing and very little air to propel the powder. This was a quantum leap for powder delivery and application equipment—providing higher transfer efficiency with less overspray—allowing powder coaters to increase line speeds and, in some cases, even eliminate manual gun stations. Not visible to the naked eye is the ability of dense phase technology to charge powder better due to the lower powder velocity and the increased relaxation time of the corona charge. In other words, more powder at a lower velocity means that more powder has a greater time to attract a charge as it passes through the corona field in front of the powder gun. When compared head-to-head with a manual spray gun and a venturi pump, the dense phase technology gun and pump will apply more powder, with fewer strokes, in a shorter amount of time. 161

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