Visual Encyclopedia of Chemical Engineering
Powder Coating
In powder coating, a dry polymer powder is applied to an object. Powder coating is less
harmful to the environment than traditional wet spray painting because it produces
lower VOC emissions. Typically powder coating is applied through an electrostatic spray
or a fluidized bed.
(Copyright Eisenmann Co., Deutschland)
Process Map
Electrostatic Spray
Powder particles are attracted to the object by application of a charge or directed by an
electric field.
(Copyright Eisenmann Co., Deutschland)
General Information
In the electrostatic spray process, compressed air pushes polymer powder out of a gun
and onto the part. Powder particles are attracted to the piece by electrostatics.
The pieces to be coated are usually hung from an overhead conveyor that moves through
the spraying area, oven and unloading area. In the oven, the powder coat cures to a hard
finish.
(Copyright Eisenmann Co., Deutschland)
Equipment Design
Electrostatic spray units are available in manual and automated systems. In manual
spray guns , such as the one shown below on the left, compressed air is blown into the
powder canister to stir the powder particles. The air and particles travel through flexible
tubing to the gun, and are then sprayed onto the piece , as shown below on the right. In
other manual systems the powder is loosened by stirring it within a hopper or vibrating
it in a container.
(Copyright Nordson Corporation., Westlake, OH)
(Copyright Eisenmann Co., Deutschland)
In automated systems the part is usually hung from an overhead conveyor that runs
through the spray booth and curing oven. Once the piece enters the booth a sensor trig-
gers the powder guns, which coat the part. Air is pulled into the booth through the open-
ings by an exhaust fan, preventing any powder from escaping. The powder is recovered
from the air by cyclones , cartridge filters , or a combination of the two. Cyclones sepa-
rate the powder from the air through centrifugal force. Air exits out the top and the pow-
der leaves at the bottom. For more information see the Cyclones section of the
encyclopedia. A cartridge filter separates by having the polymer powder collect on the
outside of the filters and the clean air passes through the filters. For more information
see the Filters section of the encyclopedia. Powder is then recycled back to the guns and
clean air is vented to the room.
(Copyright Eisenmann Co., Deutschland)
Two common spray guns are the corona and tribo guns. The corona gun creates a strong
electric field between the ionizing tip inside the gun and the piece, as shown below. Pow-
der particles passing through the ionization field become negatively charged and repel
each other, creating a fine mist. The particles follow the field lines to the piece, coating
the front, back and sides. Powder particles are more attracted to uncoated areas of the
object. The final coating thickness depends on the applied voltage. The inside section of
the piece is not coated because there is no electric field there. This phenomenon is called
the Faraday cage effect.
Instead of applying an electric field, the tribo gun mechanically applies a charge directly
to the powder particles. Friction supplies the charge when incoming particles are
deflected and forced to rub against the lining of the gun. Due to the absence of an exter-
nal electric field, particles are able to penetrate areas difficult to reach with a corona
gun.
Usage Examples
Electrostatic sprays can be used on almost any part that is painted by traditional wet
paint methods. In addition to being less harmful to the environment than wet paint, elec-
trostatic spray systems recover and recycle more than 95% of the material used.
Common substrates include steel, aluminum, zinc, lead, copper, brass, glass, and alloys.
Coating materials used include nylon, vinyl, polyolefins, epoxy, polyester, and polyeth-
ylene.
( Copyright Nordson Co., Westlake, OH)
Advantages
Disadvantages
• Polymer powders contain less than one percent VOC's.
• Provides abrasion and/or corrosion resistance.
• Electrical and thermal insulation of pieces.
• Produces thin, even coatings from 0.001" to 0.010".
• Recovery and recycle of powder is upwards of 95%.
• No solvents needed for clean up.
• Plastic spray booths repel powder particles.
• Complex parts difficult to coat using corona gun due to Faraday cage effect.
• Poor edge coverage with thin coatings.
• Polymer powder sticks to steel spray booths.
• Plastic spray booths expensive.
• Tribo gun affected by high humidity.
• More equipment required for high line speeds with tribocharging.
Fluidized Bed
General Information/Equipment Design
In fluidized bed coating, air flows upwards through a bed of polymer particles. The bed
is lifted by the air, creating a fluid-like cloud of particles. A heated object is then dipped
into the cloud. Particles that contact the object fuse to the exposed surface, coating the
object. The thickness of the coat is determined by the amount of time the object spends
in the bed.
Usage Examples
A fluidized bed process is used when a heavy coating (0.01 to 0.04 inches) is needed.
Fluidized beds also provide excellent edge coverage and are an economical way to pre-
vent corrosion by encapsulating parts. A fluidized bed is made up of a powder container
and an air receiver that are separated by a porous fluidizing bottom.
Advantages
Disadvantages
• Superior edge coverage.
• More cost effective than electrostatic spray.
• Corrosion resistance.
• Total encapsulation most effective use.
• Selective deposition of powder is difficult to achieve.
• Frequent addition of powder needed to prevent stratification of bed and poor
coat quality.
Acknowledgements
Eisenmann Co. , Deutschland
Nordson Corporation , Westlake, OH
References
Hughes, J. F. Electrostatic Powder Coating . New York: John Wiley & Sons, 1984.
Lehr, William D. Powder Coating Systems . New York: McGraw-Hill, 1991.
Miller, Emery P., and David D. Taft. Powder Coating . Dearborn, MI: Society of Manufac-
turing Engineers, 1974.
Rosen, Stephen L. Fundamental Principles of Polymeric Materials . New York: John Wiley
& Sons, 1982.
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