Metal Finishing Guide Book

2013

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Fig. 4. Hopper designs. delivery system consists of a feed hopper, a powder pump, and a powder feed hose. The feed hopper can be one of two types (see Fig. 4). The first type is called a gravity feed hopper. As the name suggests, this feed hopper uses gravity to move powder to the powder pump located at the bottom. This hopper usually is conical in shape to funnel powder to the pump. Sometimes a mechanical stirrer or vibratory assist is used to maintain an even powder flow. Frequently, without a mechanical assist, powder will bridge across the bottom of the funnel causing uneven feed to the pump. Since there is no air mixed with the powder in the hopper, this device is often employed when spraying blended metallic powders that can be stratified within a fluidized hopper. The second type of powder feed hopper uses a fluidized bed. It is the same as the fluidized bed system described previously. A compressed-air supply is connected to the plenum chamber below the fluidizing plate. The fluidizing plate causes the air to fluff powder in the hopper to a state resembling water. Now the powder can be drawn out by the powder pump. Since powder is mixed with compressed air from the plenum, the powder within this device is very homogeneous in nature. Powder pumps are mounted on the hopper and are connected to a pick-up tube to draw powder out of the hopper. These pick-up tubes usually are positioned an appropriate distance into the fluidized bed to assure that the turbulence usually present on the surface of the fluidized plate is not drawn up into the powder pump. This turbulence can cause inconsistent powder feed to the applicators. Box Feeders Powder equipment manufacturers also provide methods of pumping powder coatings directly from their shipping containers (box or bag) to the spray gun. This method is called the box feeder and utilizes a tilted vibrating table to support the box of powder. A powder pump connected to a pick-up tube is inserted in the lowest portion of the box. A compressed air jet is employed at the end of 212

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