TRIBO OR CORONA? HERE'S HOW TO DECIDE

Alan J. Knobbe Senior Project Eng i neer

Nordson Corporation Amherst, Ohio

Presented at: "Finishing '93 Conference and Exposition'' October 25-28, 1993 Dr. Albert B. Sabin Convention Center Cincinnati, Ohio

Copyrighted by SME; Technical Paper Number t o be Assigned

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INTRODUCTION

There are many important decisions to be made and factors to consider by a company investigating the purchase of a new powder system or by a company presently spraying powder and looking to upgrade their system. What equipment manufacturer should the equipment be purchased from? What will it cost? What kind of service and support will be provided? What equipment is the best value for the money. The questions go on and on. Over the years, significant improvements have been made to both powder coatings and to the equipment for delivering, spraying and recovering them. This may further cloud the decision. A very important, fundamental question is: What equipment will best coat the product to specifications? The answer to this question and to the preceding questions may be influenced to a large extent by the type of system it is - tribo or corona. But how do you decide which one is best for your company? First of all, it’s important to understand the basic principles of each type of equipment.

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BASIC PRINCIPLES

Corona charging guns work by bombarding powder particles sprayed from the gun with charged particles called ions. The corona charging process is illustrated in Figure I , Figure 3 contains a photo of a state-of-the-art corona gun in operation.

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The corona charging process begins with a potential or voltage applied to one or more electrodes near the front of the gun. A high-voltage generator is used to produce this voltage up to 100,OOO volts! The entire high-voltage generator may be located away from the gun in the gun control module or altematively, part of it may be located within the gun body itself. For spraying most types of finishing powders, a negative polarity voltage is produced in the generator and results in the powder particles accumulating a negative charge. Positive polarity generators are also typically available as an option and are used primarily for charging nylon powders.

As the voltage on the electrode is increased, an electric field is produced between the gun and grounded part. When the electric field in the vicinity of the electrode reaches a strength of about 30 kV per centimeter, the field is strong enough to break down the air in the vicinity 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 ions and accumulate a negative charge. These charged powder particles are influenced by the electric field between the gun and part and tend to follow the the electric field to the part as represented by lines in Figu. 7 . Some powder particles may be shielded from other particles in the charging zone and therefore, not accumulate a charge. For these particles, aerodynamic forces might propel them toward the part.

Ions which don’t become attached to powder particles in flight are known as excess ions or fiee ions. Free ions, being charged particles, also tend to follow the electric field and many are deposited onto the part. Generally, the majority of ions produced do not attach to powder particles.

Tribo-charging guns charge powder particles as a result of the intimate contact and subsequent separation of the powder particles from the gun walls. The tribo charging process is illustrated in figure 2- figure 4 contains a photo of a state-of-the-art tribo gun in operation.

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3 When two different materials are brought into contact, there will be a transfer of

charge from one material to the other in order to eliminate the imbalance of charge. The magnitude and direction of the charge transfer depends on many factors, including the chemical and electronic structure of both materials. Over the years, a lot of testing has been done contacting one material against another and measuring the resulting magnitude and polarity of the charge attained on each of the materials. So called "Triboelectric Series" have been published which show various materials and their affinity for accumulating either a negative or a positive charge.

Polytetrafluoroethylene or FTFE is typically used as the powder contact walls in a tribo gun. Powder particles, of course, are a composition of materials comprised of resin, pigment, fillers, and possibly other additives. Experience has shown that most finishing powders become positively charged as a result of their contact with PTFE. Theoretically, the gun walls will be left with a charge equal in magnitude but opposite 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 which would cause the gun to stop charging.

Tribo guns charge the powder particles within the gun as long as the powder particles contact the PTFE walls. The more contacts a powder particle makes with the walls and the harder it hits them, the greater the charge on the particle. As long as there's PTFE left in the gun for the powder to contact, the powder will get charged.

Note from figure 2 that the tribo gun does not have an electrode at high potential or the resulting electric field between the gun and part like a corona gun has. Therefore, the airflow from the gun plays a more significant role with a tribo gun transporting the powder particles onto the part.

It's important to realize that every powder system has a combination of both tribo and corona charging going on. Every time a powder particle makes contact with any surface in a powder system - in the hopper, feed hose, gun, booth or sieve - it could be either accumulating or losing some of its charge by tribo charging. Furthermore, the voltage can be high enough and an edge sharp or pointed enough on many components in a powder system to result in a field strength high enough to ionize the air and form a corona discharge. Systems are referred to as either tribo or corona usually because of the dominant charging mode designed into the spray guns.

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EQUIPMENT DESIGN DIFFERENCES

The different charging principles result in many differences in the design of the spray equipment and the controls.

Tribo EauiDment

1. The powder contact surfaces in the gun are designed to maximize tribo charging powders and wear-life and to minimize impact fusion. Some sacrificed wear is a necessary condition for an effective overall design.

2. Many different sprayhead and nozzle designs are possible since the powder is already charged by the time it enters the sprayhead. Sprayhead design is greatly simplified without the requirements for the charging electrode of a corona gun.

3. The powder flow rate and the velocity of the powder are adjusted in order to adjust the charging of the powder. Lower powder flow rate and higher velocity results in better charging.

4. Tribo systems have no high voltage power supplies or electrostatic cables.

5. Tribo guns have a large surface area made out of PTFE for the powder to contact in order to maximize the possibility of the powder contacting the walls for charging. Therefore, a tribo gun may be both larger and heavier than a corona - particularly a corona gun without the high voltage generator in the gun.

Corona EauiDment

1. The powder contact surfaces in the gun are designed to minimize tribo charging of powders, impact fusion, and wear-life.

2. Sprayhead design is very dependent on the location of the charging electrode(s). This limits the number of designs available and makes developing new ones more difficult.

3. Charge on the powder particles is adjusted primarily by adjusting the voltage output of the high voltage generator and also by adjusting the powder flow rate. Higher voltage (up to around 1OOkV) and lower powder flow rates result in better charging.

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4. Corona systems have a high voltage generator in the gun or the gun control unit or a combination of both. If the high voltage generator is in the control unit, there will be a high voltage electrostatic cable to deliver the voltage from the generator to the gun.

5. The powder "wetted" surface area can be minimized in a corona gun and the parts don't necessarily have to be made out of PTFE. Therefore, a corona gun may be smaller and lighter than a tribo gun.

SPRAYING ENVIRONMENT DIFFERENCES

The different charging principles also result in many differences in the spraying environment.

Tribo SRravine Environment

1. There is only a weak electric field between a tribo gun and the part to be coated. Airflow and the charge on the powder particles is responsible for getting the particles onto the part.

2. There is no electrode generating free ions for charging the powder or for building up additional charge on the surface of the part.

3. Tribo guns can have excellent control of the transport of the powder particles onto a part. Because of many options available with the design of sprayheads and nozzles, the transport of the particles can be maximized to deliver them at just the right quantity, in the right direction, and at the right velocity for coating a part.

4. Powder charging is largely independent of gun-to-part distance when spraying with a tribo gun. Therefore, a tribo gun may be positioned croser to a part and the sprayhead and nozzles may make up a large part of the distance between the gun and the part.

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There can be a very strong electric field between a corona gun and the part to be coated. Therefore, in addition to airflow and the charge on the powder particles, the force acting on the powder particle from the electric field can be a significant force for transporting and depositing the powder particles onto the part.

There can be a significant charge directed onto a part by the free ions which do not attached onto powder particles. This ion current can also accumulate on other objects in the spray environment. This makes it very important to ensure all objects in the spray environment are well grounded electrically.

Sprayhead design is coupled with the arrangement of the charging electrode(s). This limits the design options for sprayheads and nozzles and results in weaker control of the powder transport function.

Powder charging can be very dependent on the gun-to-target distance. This is dependent on the design of the high voltage generator. Current between the gun and part increases as the gun is brought closer to the part. Higher current results in lower voltage on the charging electrode and hence, a lower electric field. A lower electric field results in poorer charging.

SELECTION CRITERIA

Part Geo metrv

Tribo-charging guns can effectively coat the widest variety of parts. Since a tribo gun doesn’t have a strong electric field between the gun and part like a corona gun may have, powder sprayed from a tribo gun can easily penetrate into Faraday cage areas on parts. Because of the wide variety of sprayhead and nozzle combinations possible, a tribo gun can have the optimum control of the powder sprayed: excellent control of the quantity of powder sprayed and its direction and velocity. As a result of t h i s control, many parts can be coated automatically without any manual gun touch-up. Excellent parts for coating with tribo gun include condensers, transformers, bicycle or motorcycle frames, wire baskets, folding chairs and others.

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Part Ma terial

A tribo-charging gun can coat some parts which are not highly electrically conductive without the need for the prior application of a conductive coating. This is primarily because a tribo gun does not deposit a large quantity of free ions on a part like a corona gun might. The parts stil l need to be electrically grounded, but the surface conductivity of the part material or of the powder coating may be large enough for effective grounding. Spray trials performed as close as possible to the actual production conditions are necessary to verify the coatability of non- metallic parts.

Tribo guns may also be better 'for recoating parts compared to a corona gun, again, because of the slow build-up of charge on the part.

Coating Apwarance

The appearance of the powder coating on a part can be affected by the bombardment and deposition of the free ions from a corona gun. Since there are comparatively few free ions produced by a tribo gun, the resulting coating can be particularly smooth and blemish-free. A good application for tribo guns is applying a powder clear coat onto automobile wheels.

Coating Thickness

Corona guns are particularly good at applying thin films consistently onto parts. However, as coating thickness increases, charge can build up on a part from both charged powder and free ions. This charge can repel incoming powder particles and effectively self-limit the coating thickness. Tribo guns, however, producing few free ions, can be an excellent choice for applying thick films. A film build of several hundred microns on an unheated part in a single pass is possible with a tribo gun.

GoatinP - Uniformity

The strong electric field from the high voltage electrode on a corona gun can cause powder particles to concentrate on part surfaces closest to the gun and on corners and edges. A tribo gun's comparatively weak electric field does not cause these variations in coating thickness. Furthermore, coating uniformity might be better optimized with a tribo gun through the selection of the best sprayheadhozzle combination for coating a particular part.

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CoatinP ComDosition

In general, corona charging is considered to be insensitive to the composition of powder coatings, whereas tribo charging is dependent on the composition of the surface of the powder particles. Finding a wide variety of powders which tribo- charged acceptably used to be a problem, however, the technology for manufacturing powders which work well in a tribo system has improved greatly in the last several years. Most powder manufacturers today formulate powders specifically for tribo application. Before finalizing your selection of a tribo system, spraying trials are highly recommended with potential powders before selecting one for coating your parts.

The abrasiveness of the powder can also be an important consideration. Charge accumulates on a powder particle as a result of its contact with the walls in the tribo gun. Harder contacts at higher velocity can result in better charging, but it can also result in higher wear rates on the powder contact parts. Frit (powdered porcelain enamel) is never sprayed with a tribo gun. Metallics are usually sprayed with a corona gun, however, some are sprayed well with a tribo gun.

Line Speed

In the past, one of the disadvantages of tribo guns was the relatively low powder output compared to corona guns. This limited tribo guns in some cases to applications with small parts and low line speeds. This limitation no longer exists. Tribo guns are available today which have the same powder flow rate capability as corona guns.

Color ChanFe

The wetted surface (the area contacted by the powder) may be greater in a tribo gun compared to a corona gun. Therefore, color change may take longer with a tribo gun because of the larger surface requiring cleaning. Of course, how easily a gun can be disassembled is also an important factor in color change time.

Eauipment Construction

From a manufacturing standpoint, tribo equipment can be considered more robust, and it can be more reliable than corona equipment. Tribo equipment is simpler since it does not have a high voltage power supply or cable and therefore, has fewer things which can go wrong with it.

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Spraviny Process Monitor

The current generated in a tribo gun is typically displayed on the control console and can be used to help an operator monitor the performance of the equipment and spraying process. The current generated is proportional to the powder flow rate. Therefore, a reduction in the current can indicate a reduction in the powder flow rate and an operator can be notified of this condition to find and correct the problem immediately

CONCLUSION

Clearly, the decision between tribo and corona equipment is not always an easy one. Only by considering the parts to be coated and the coating specifications, understanding the differences between the equipment, and considering the technical criteria presented in this paper can the best system be selected.

BIBLIOGRAPHY

Hughes, J.F., Electrostatic Powder Coating, Research Studies Press Ltd., 1984

Moore, A.D., Electrostatic and Its Applications, John Wiley and Sons, Inc. 1973

FIGURE 3 STATE-OF-THE ART CORONA GUN

FIGURE 4 STATE-OF-THE ART TRIBO GUN

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