Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 172 of 331

tron donor. According to this study, all materials that contact PTFE should be- come positively charged. PTFE is typically used for the powder contact walls in a tribo gun. Powder par- ticles, of course, are a composition of resin, pigment, fillers, and possibly other additives. Experience agrees well with this series in that most finishing powders become positively charged as a result of their contact with PTFE. Today, most pow- der manufacturers formulate powders specifically for tribo guns. Tribo guns charge powder particles as long as the powder particles contact the PTFE gun walls. Compared with corona charging, tribo charging is a high- ly efficient charging process. The more contacts a powder particle makes with the walls and the harder it hits them, the greater the charge on the particle. The- oretically, the gun walls will be left with a charge equal in magnitude but op- posite in polarity to the charge accumulated on the powder particles. This charge on the gun walls must be conducted away or else it will build up inside the gun and the gun will stop charging. Tribo guns are also available with optional powder-contact parts made out of nylon 6/6. Because of its location in the triboelectric series, nylon parts are ide- al for charging PTFE powders. In this case, the PTFE powder particles become neg- atively charged and the nylon gun walls become positively charged. Since a tribo gun does not rely on a high-voltage generator or an electrode at high potential, there is only a weak electric field between a tribo gun and the work- piece. The airflow from a tribo gun thus plays a significant role in transporting the powder particles to the workpiece. The spray head can also play a significant role in the performance of a tribo gun. Since the powder particles are already highly charged by the time they en- ter the spray head, many spray head designs are possible for optimizing the transport of the particles in just the right quantity, in the right direction, and at the right velocity, onto a workpiece.Tribocharging guns can have a very high transfer efficiency and they can effectively coat the widest variety of workpieces. They are particularly good at coating difficult-to-coat workpieces, which have deep recesses and many Faraday cage areas. 171

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