New methods of improving Faraday-cage coating, finish quality and uniformity, and recoating operations.
Sergey Guskov Powder Systems Group Nordson Corporation
Published by Nordson Corporation
Introduction Over the past decade, worldwide popularity of powder coating technology has enjoyed steady, double-digit growth. One of the forces drivingthis tremendous success has been continuous improvements in application equipment used in the powder coating process. Since the early days of powder coating, powder coaters and application equipment manufacturers have faced several challenges, including: maximization of first-pass transfer efficiency; effective coating of Faraday-cage areas; better finish quality and uniformity; and recoating ofrejected parts. Recent technological developments, however, have allowed leading equipment manufacturers to offer users new equipment features that more closely meet these challenges. An understanding of the electrostatic phenomena involved in the powder coating process is equally important to equipment manufacturers and users. As equipment becomes more and more sophisticated to offer morecapabilities previously unavailable, it is important that powder coaters understand what features are worth the investment, in which applications, and why. This paper focuses on corona-charging application systems, and presents an overview of electrostatic processes utilized in powder coating technology in light of new equipment features now available.
This paper was presented at the Powder Coating ’96Conference in Indianapolis, Indiana, September 18, 1996. It is published in the conference proceedings under the original title, Application of Powder Coatings: Old Problems, New Findings, New Developments, and is covered by copyright protection.
Corona Development In corona-charging systems, a sharply non-uniform electric field is created between a gun and part by applying high (usuallynegative) voltage potential to a pointed electrode. Non-uniformity of this field is imperative since the field lines converge on sharp points and the density of field lines in any area represents the strength of the electric field. Therefore, if we apply a high-voltage potential to a single-point electrode and position a larger-size grounded object before the electrode, we will create an electric fieldwhose strength is greatest at the tip of the pointed electrode. There are always free electrons or ions present in the air. If an electron passes through a strong electric field, it will start moving in this field along the field lines and be accelerated by the field force. As the electron accelerates along the field lines, it will ultimately run into an air molecule (see Figure 1). If the fieldstrength is adequate and the electron has gathered sufficient kinetic energy while traveling along the field lines, its impact on the air molecule will be strong enough to split that molecule to form two secondary electrons and one positive ion (the remainder of the molecule). Secondary electrons will instantly be accelerated in the electric field. Moving along the field lines, they will split newmolecules and create more ions and electrons. Figure 1
This process is corona discharge. It is selfsustaining at field strengths equal to, or greater than, some starting level. Immediately after the ionization process begins, the space between the spray gun and grounded part becomes filled with millions of ions and free electrons. Henceforth in this paper, the term “free electrons” will bereplaced with the more commonly used term “free ions1.” Charging Powder Particles Figure 2 illustrates a powder particle in an electric field. The uncharged dielectric particle will distort the external electric field so that some field lines will extend toward the particle’s surface, enter at a 90º angle, pass through it, and exit at a 90º angle. If free ions are present in the external electric...