Metal Finishing Guide Book


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Fig. 5. Powder pump. est portion of the box. A compressed air jet is employed at the end of this tube to assist powder flow into the tube. Powder is then pumped directly from the box to the spray gun without the need of a feed hopper. This approach makes color change cleanup quick and easy, as only the pick-up tube, pump, and hose need to be cleaned. Changing the powder box completes the color change task. Pumps Most powder pumps are designed to work by the venturi principle. Compressed air is directed perpendicular to the venturi pickup, causing a differential in pressure, or vacuum, that siphons powder out of the feed hopper or box feeder. When the powder enters the compressed air stream, it is pushed through the powder hose toward the applicator. An additional compressed air supply is introduced at the point where the powder enters this air stream (see Fig. 5) to dilute the powder and increase its velocity. Increasing powder velocity ensures that the powder stays within the air stream as it proceeds through the hose, reducing surging or pulsing problems. Surging occurs when the powder lays at the bottom of the hose until enough air pressure builds behind it to push it out with a burst. Both air supplies have check valves to force the air to go through the powder hose, allowing independent control of both powder quantity and speed through the feed hose. Powder hose can be made from several materials, including urethane, vinyl, and certain rubber compounds. Hose diameter and length are critical. Diameter is dictated by the powder pump used; it always should match the manufacturer���s recommendation. Length always should be as short as possible to reduce back pressure to the powder pump. This reduces surging of the powder stream to the gun. Avoid bends and kinks in the hose routing. The more powder you pump using venturi style pumps the faster it travels through the electrostatic field. Consequently, transfer efficiency will be lower at higher feed rates. Applications requiring highly controlled powder flow at a wide range of output rates use high density - low pressure (HDLP) powder pumps. These 247

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