TRIBO OR CORONA? HERE'S HOW TO DECIDE
Alan J. Knobbe
Senior Project Eng ineer
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 F i g u . 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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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.
1.
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.
2.
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.
3.
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.
4.
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 Geometrv
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 Material
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 still 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 nonmetallic 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 tribocharged 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
NOTES
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NOTES