Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 161 of 331

Fig. 4. Hopper designs. Controlling these variables is important to assure that the powder particle will be properly charged. Remember: if the powder is not charged, it will not adhere to the part unless the part is hot enough for the powder to stick on contact. The amount of electrostatic charge that typically is developed by this apparatus is less than that produced by corona equipment. The polarity of the tribo charge is a function of the material being sprayed and the material that it is rubbed against. If the same two materials are used, the polarity will always be the same. Tribo-charge applicators can often be used to overcome Faraday areas on dif- ficult-to-coat parts, as there is no electrostatic field used to charge the powder. This flexibility, however, is often overshadowed by the additional process and coat- ing materials controls that are required to ensure successful coating. Powder Bells This device uses an air turbine to rotate a conical cup used to atomize the pow- der coating. Powder is pumped to the cup where the rotational forces cause complete powder atomization. The feed system used to support this device is sim- ilar to that of spray guns. These devices employ the corona charging method, de- scribed earlier in this article. Powder bells are capable of dispersing a large quantity of powder coating over a large area. Therefore, the typical applications for this device are large flat com- ponents, such as appliances and automobile bodies. POWDER DELIVERY All spray application equipment requires a delivery system (see Fig. 3). This de- livery 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 160

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