Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 137 of 331

Summary Of the three methods discussed for spraying waterbornes electrostatically all have their advantages and disadvantages. The end user must decide as to which method is best suited for their application. Voltage blocks are the simplest and can be used with any type of fluid supply, but up front cash can sometimes be a factor in the mind of the decision maker. Isolated systems can be cheaper on most occasions, but can also take up a lot of valuable floor space. Isolated systems are also the least safe and may be impractical when your fluid supply is a remote paint kitchen. Although indirect charging may be the least efficient of the three methods discussed, it may be the most practical in some applications. For example, in automotive assembly plants where a large paint kitchen is involved or extremely fast color changes are nec- essary. Voltage blocks and isolated systems have been proven to provide higher trans- fer efficiencies. ELECTROSTATIC PROCESS FOR PLASTICS & OTHER NONCONDUCTIVE SUBSTRATES The ideal application for the use of electrostatics is metal because the only thing that needs to be done to spray electrostatically is to connect a ground wire to the product; however, when you try to electrostatically spray a nonconductive substrate, such as plastics, it must be made conductive. There are several ways of making the part being coated or the application conductive. The most common of these being: 1. Build a bracket of grounded metal and place the nonconductive part between the applicator and the conductive fixture.(The charged particles will see the ground and be drawn to the part being coated. Examples for utilizing this method of tech- nology would be the coating of fabrics, paper or other thin structures.) 2. Certain materials become conductive with heat. Materials such as glass, rubber products, and some plastics may be heated until they are conductive and electrosta- tically sprayed while warm. 3. All nonconductors, such as wood, rubber, plastic and glass, may also be treat- ed with chemical sensitizers. These are generally hydroscopic chemicals that attract moisture onto the surface of the product to create conductivity. Controlled concen- trates of the sensitizer may be applied by dipping, wiping, spraying or a mist cham- ber. After treatment, the part becomes conductive when exposed to adequate hu- midity such as a humidity chamber or high ambient humidity (70% relative humid- ity). Sensitizers are non film-forming liquids. 4. Another method of making the part conductive is by using a conductive primer. The conductive primer can be applied to the substrate by conventional means, thus allowing the top coat to be applied electrostatically. Conductive primers may be sprayed, dip coated, flow coated, or molded in. The reason for making nonconductive parts more acceptable to an electrostatic charge is to utilize the most efficient process with the highest quality finish at the most minimal cost. By utilizing the electrostatic process, you will achieve each of these benefits. COST SAVINGS Transfer Efficiency/Paint Savings The cost savings associated with the use of electrostatic equipment can be realized in many different areas. The most obvious savings is in paint usage. With the increase of high-solids,plural-component, and base/clear finishes, it is not unrealistic to pay 136

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