Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Fig. 6. Gun motion devices. 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 pres- sure, 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 in- troduced at the point where the powder enters this air stream (see Fig. 5) to di- lute 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 dic- tated by the powder pump used; it always should match the manufacturer's rec- ommendation. Length always should be as short as possible to reduce back pres- sure 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 devices deliver a column of powder to the applicator without hav- ing to mix it with compressed air. Reducing the compressed air within the pow- 162

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