coating materials and 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 applicationof powder coating materials. They are corona guns and tribo guns, where coronaand tribo refer to the predominant process used in the guns for electrostaticallycharging the powder particles.

CORONA GUNS

Corona charging guns work by bombarding powder particles sprayed fromthe gun with charged particles called ions. The corona charging process isillustrated in Figure 1.The corona charging process begins with a potential (or voltage) applied toone or more electrodes at or near the front of the gun. A high-voltage generatoris used to produce this voltage of up to 100,000 V. As the voltage on the electrodeis increased, an electric field is produced between the gun and the groundedworkpiece. When the electric field in the vicinity of the electrode reaches astrength of about 30,000 V/cm, the field is strong enough to break down theair in the vicinity of the electrode. This electrical breakdown of air results in thecreation of charged molecules or ions in the form of a continuous dischargeknown as a corona discharge. Powder particles exiting the gun travel near theelectrode where they are bombarded by these ions and accumulate a charge.Both the charged ions and the charged powder particles are influenced by theelectric field between the gun and the workpiece and tend to follow the electricfield to the part, as illustrated by lines in Figure 1. Ions that do not becomeattached to powder particles in flight are known as excess ions or free ions.Typically, only a few percent of the ions generated actually become attached topowder particles in flight. Some powder particles may be shielded from otherparticles in the charging zone and, therefore, do not accumulate a charge. Forthese particles, aerodynamic forces resulting from the powder conveying airmight propel them toward the workpiece.Charged powder particles and excess ions are both deposited on the groundedworkpiece. The charged powder particles are held onto the workpiece electrostaticallyuntil it is transported into an oven for curing. Heat causes the powderparticles to flow together and fuse into a continuous film.The Pauthenier equation describes the charge, over time, accumulated by apowder particle exposed to a corona discharge:

Q(t) =Ar2Et/(t+t)

where, A = a constant, which depends on the particle composition, r = particleradius, E = electric field strength = electrode voltage/gun-to-workpiece distance,t = time, and t = charging time constant.The charging performance of a corona gun can be affected by the gunto-workpiece distance. Today, corona guns are available that use speciallydesigned high-voltage generators or gun-control modules to reduce or eliminatethis dependency.The high-voltage generator may be located remotely from the gun in thegun-control module or, alternatively, part of it may be located within the gunbody itself. When the high-voltage generator is located in the gun-controlmodule, a high-voltage cable is used to transmit the power to the gun. Whena portion of the high-voltage generator is located within the gun body, a lowvoltagecable is used.For spraying most types of finishing powders, a negative-polarity voltage isproduced in the high-voltage generator.This results in the powder particlesaccumulating a negative charge. Positive-polarity generators are also typicallyavailable as an option and are used primarily for charging nylon powders.Two basic types of spray heads are available for shaping the powder particlesinto a cloud as they exit a corona gun. They are called conical deflectors and flatspraynozzles. Conical deflectors shape the powder cloud into a circular, hollow,dome-shaped pattern. These spray heads can produce a large, low-velocity spray,360° SD in circumference. They are best for simple-shaped workpieces and canproduce a very high transfer efficiency. Flat-spray nozzles typically have a singleslot through which the powder particles exit. The resulting powder cloud is fanshapedfrom the side, but has a narrow width. These nozzles may have a highervelocity than a conical spray head and are, therefore, best for spraying parts withdeep recesses and corners.Many equipment manufacturers design their electrostatic corona powderspray guns to comply with the different codes governing the manufacture anduse of these products worldwide. Some of the worldwide agencies that testand issue approvals on these spray guns are Factory Mutual (United States),Canadian Standards Association, European Committee for ElectrotechnicalStandardization (CENELEC), and the Research Institute of Industrial Safety(RIIS, Japan). In the United States, a local fire marshal would typically look for“Approved” equipment or compliance with National Fire Protection Association(N.F.P.A.) Standard 33, Spray Application Using Flammable and CombustibleMaterials, before permitting an installation to start production.

TRIBO GUNS

Tribo or triboelectric charging guns charge powder particles as a result of theintimate contact and subsequent separation of the powder particles from thegun walls. The word “tribo” comes from the Greek word tribein meaning to rub.The tribo charging process is illustrated in Figure 2. Charged powder particles and excess ions are both deposited on the groundedworkpiece. The charged powder particles are held onto the workpiece electrostaticallyuntil it is transported into an oven for curing. Heat causes the powderparticles to flow together and fuse into a continuous film.The Pauthenier equation describes the charge, over time, accumulated by apowder particle exposed to a corona discharge:

Q(t) =Ar2Et/(t+t)

where, A = a constant, which depends on the particle composition, r = particleradius, E = electric field strength = electrode voltage/gun-to-workpiece distance,t = time, and t = charging time constant.The charging performance of a corona gun can be affected by the gunto-workpiece distance. Today, corona guns are available that use speciallydesigned high-voltage generators or gun-control modules to reduce or eliminatethis dependency.The high-voltage generator may be located remotely from the gun in thegun-control module or, alternatively, part of it may be located within the gunbody itself. When the high-voltage generator is located in the gun-controlmodule, a high-voltage cable is used to transmit the power to the gun. Whena portion of the high-voltage generator is located within the gun body, a lowvoltagecable is used.For spraying most types of finishing powders, a negative-polarity voltage isproduced in the high-voltage generator. This results in the powder particlesaccumulating a negative charge. Positive-polarity generators are also typicallyavailable as an option and are used primarily for charging nylon powdersTwo basic types of spray heads are available for shaping the powder particlesinto a cloud as they exit a corona gun. They are called conical deflectors and flatspraynozzles. Conical deflectors shape the powder cloud into a circular, hollow,dome-shaped pattern. These spray heads can produce a large, low-velocity spray,360° SD in circumference. They are best for simple-shaped workpieces and canproduce a very high transfer efficiency. Flat-spray nozzles typically have a singleslot through which the powder particles exit. The resulting powder cloud is fanshapedfrom the side, but has a narrow width. These nozzles may have a highervelocity than a conical spray head and are, therefore, best for spraying parts withdeep recesses and corners.Many equipmentmanufacturers design their electrostatic corona powderspray guns to comply with the different codes governing the manufacture anduse of these products worldwide. Some of the worldwide agencies that testand issue approvals on these spray guns are Factory Mutual (United States),Canadian Standards Association, European Committee for ElectrotechnicalStandardization (CENELEC), and the Research Institute of Industrial Safety(RIIS, Japan). In the United States, a local fire marshal would typically look for“Approved” equipment or compliance with National Fire Protection Association(N.F.P.A.) Standard 33, Spray Application Using Flammable and CombustibleMaterials, before permitting an installation to start production.

TRIBO GUNS

Tribo or triboelectric charging guns charge powder particles as a result of theintimate contact and subsequent separation of the powder particles from thegun walls. The word “tribo” comes from the Greek word tribein meaning to rub.The tribo charging process is illustrated in Figure 2.When two different materials are brought into contact, there will be a transferof charge from one material to the other in order to eliminate the imbalance ofcharge. The magnitude and direction of the charge transfer depends on manyfactors including the chemical and electronic structure of both materials. Overthe years, a lot of testing has been done contacting one material against anotherand measuring the resulting magnitude and polarity of the charge attained oneach of the materials. The results of one such study, presented in the form ofa so-called “triboelectric series,” are shown in Table I. The farther two materialsare away from each other in the series, the greater the triboelectric chargethat should be produced when these two materials come in contact. Note thatpolytetrafluoroethylene (PTFE) is shown at one end of the series as being thestrongest electron acceptor and nylon 6/6 is shown at the other end as beingthe strongest electron donor. According to this study, all materials that contact

Electron Donor (+)

Table I. Triboelectric Series

PTFE should become positively charged.PTFE is typically used for the powder contact walls in a tribo gun. Powder particles,of course, are a composition of resin, pigment, fillers, and possibly otheradditives. Experience agrees well with this series in that most finishing powdersbecome positively charged as a result of their contact with PTFE. Today, mostpowder manufacturers formulate powders specifically for tribo guns.Tribo guns charge powder particles as long as the powder particles contactthe PTFE gun walls. Compared with corona charging, tribo charging is a highlyefficient charging process. The more contacts a powder particle makes withthe walls and the harder it hits them, the greater the charge on the particle.Theoretically, the gun walls will be left with a charge equal in magnitude butopposite in polarity to the charge accumulated on the powder particles. Thischarge on the gun walls must be conducted away or else it will build up insidethe gun and the gun will stop charging.Tribo guns are also available with optional powder-contact parts made outof nylon 6/6. Because of its location in the triboelectric series, nylon parts areideal for charging PTFE powders. In this case, the PTFE powder particles becomenegatively charged and the nylon gun walls become positively charged.Since a tribo gun does not rely on a high-voltage generator or an electrodeat high potential, there is only a weak electric field between a tribo gun and theworkpiece. The airflow from a tribo gun thus plays a significant role in transportingthe powder particles to the workpiece.The spray head can also play a significant role in the performance of a tribogun. Since the powder particles are already highly charged by the time theyenter the spray head, many spray head designs are possible for optimizing thetransport of the particles in just the right quantity, in the right direction, andat the right velocity, onto a workpiece.Tribocharging guns can have a very hightransfer efficiency and they can effectively coat the widest variety of workpieces.They are particularly good at coating difficult-to-coat workpieces, which havedeep recesses and many Faraday cage areas.