INNOVATIONS: PRESENT&FUTURE
Electrostatic Phenomena in Powder Coating: Methods of Improving Faraday-Cage Coating, Finish Quality and Uniformity, and Recoating Operations Sergey Guskov Nordson Corporation Europe
ics.eu@nordson.com
is adequate and the electron has gathered orldwide popularity of powder Corona Development sufficient kinetic energy while traveling along In corona-charging systems, a sharply noncoating as a process has the field lines, its impact on the air molecule uniform electric field is created between a gun enjoyed steady, doublewill be strong enough to split that molecule to and part by applying high (usually negative) digit growth over many years. One form two secondary electrons and one positive voltage potential to a pointed electrode. Nonof the forces driving this success has ion (the remainder of the molecule). Secondary uniformity of this field is imperative since the been continuous improvements in the electrons will instantly be accelerated in the field lines converge on sharp points and the application equipment used in the powder electric field. Moving along the field lines, they density of field lines in any area represents the coating process. Since the early days will split new molecules and create more ions strength of the electric field. Therefore, if we of powder coating, powder coaters and and electrons. apply a high-voltage potential to a single-point application equipment manufacturers Because opposite charges attract, positive electrode and position a larger-size grounded have faced several challenges, including: ions remaining from each split molecule will object before the electrode, we will create an maximization of first-pass transfer also be accelerated by the field force and efficiency; effective coating of Faradaymove along the field lines. Their motion, cage areas; better finish quality and however, will be in the opposite direction uniformity; and recoating of rejected toward the gun’s negative electrode. Once parts. Technology developments, again, if the electric field is sufficient, however, have allowed leading positive ions can either split molecules on equipment manufacturers to offer their way toward the negative electrode users new equipment features that or, if they reach the electrode, impact it so more closely meet these challenges. hard that they will split new ions from the Additionally, an understanding of the 1 metal surface of the electrode. electrostatic phenomena involved in This process is corona discharge. It is the powder coating process is equally Figure 1: A sharply non-uniform electric field is created selfsustaining at field strengths equal important to equipment manufacturers between a gun and part by applying high (usually negative) voltage potential to a pointed electrode. to, or greater than, some starting level. and users. As equipment becomes more Immediately after the ionization process and more sophisticated, to offer more electric field whose strength is greatest at the begins, the space between the spray gun and capabilities previously unavailable, it is grounded part becomes filled with millions of tip of the pointed electrode. important that powder coaters understand ions and free electrons. Henceforth in this paper, There are always free electrons or ions present what features are worth the investment, in the air. If an electron passes through a the term “free electrons” will be replaced with in which applications, and why. This paper the more commonly used term “free ions1.” strong electric field, it will start moving in this focuses on corona-charging application field along the field lines and be accelerated systems, and presents an overview 1 “Free ions” are negative ions, produced by the corona by the field force. As the electron accelerates of electrostatic processes utilized in ionization process, which do not get captured by powder particles and remain free in the space between the gun along the field lines, it will ultimately run into powder coating technology in light of new and grounded part traveling towards the closest ground an air molecule (Fig. 1). If the field strength equipment features now available. along the field lines. © Nordson
W
94
N. 47 - SEPTEMBER/OCTOBER 2017 - international PAINT&COATING magazine