Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 169 of 331

application methods POWDER SPRAY GUNS BY ALAN J. KNOBBE NORDSON CORP., AMHERST, OHIO Two basic types of electrostatic powder guns are used for the spray application of powder coating materials. They are corona guns and tribo guns, where coro- na and tribo refer to the predominant process used in the guns for electrostati- cally charging the powder particles. CORONA GUNS Corona charging guns work by bombarding powder particles sprayed from the gun with charged particles called ions. The corona charging process is il- lustrated in Figure 1. The corona charging process begins with a potential (or voltage) applied to one or more electrodes at or near the front of the gun. A high-voltage generator is used to produce this voltage of up to 100,000 V. As the voltage on the electrode is in- creased, an electric field is produced between the gun and the grounded work- piece. When the electric field in the vicinity of the electrode reaches a strength of about 30,000 V/cm, the field is strong enough to break down the air in the vicin- ity of the electrode. This electrical breakdown of air results in the creation of charged molecules or ions in the form of a continuous discharge known as a corona discharge. Powder particles exiting the gun travel near the electrode where they are bombarded by these ions and accumulate a charge. Both the charged ions and the charged powder particles are influenced by the electric field between the gun and the workpiece and tend to follow the elec- tric field to the part, as illustrated by lines in Figure 1. Ions that do not become attached to powder particles in flight are known as excess ions or free ions. Typ- ically, only a few percent of the ions generated actually become attached to pow- der particles in flight. Some powder particles may be shielded from other parti- cles in the charging zone and, therefore, do not accumulate a charge. For these Fig. 1. Corona charging process. 168

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