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Powder Coating
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These are pictures from a supposedly powdercoated sandblasting cabinet we bought.
We purchesed this unit from a local supplier here in Knoxville Tn. These scraps are from a hole we cut into the top of the cabinet to install a flourescent light.
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POWDER COATING
Mario S Pennisi
Principal
Consultant - Penlia & Co
PO Box 6198
UPPER MOUNT
GRAVATT, QUEENSLAND, AUSTRALIA 4122
Phone: +61 7
3849 6460; Fax: +61 7 3849 7302;
E-mail: mariopen@ozemail.com.au
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What is powder coating?
Powder coating
is by far the youngest of the surface finishing techniques in common
use today. It was first used in Australia about 1967.
Powder coating
is the technique of applying dry paint to a part. The final cured
coating is the same as a 2-pack wet paint. In normal wet painting
such as house paints, the solids are in suspension in a liquid
carrier, which must evaporate before the solid paint coating is produced.
In powder
coating, the powdered paint may be applied by either of two techniques.
The item is
lowered into a fluidised bed of the powder, which may or may not be
electrostatically charged, or
The powdered
paint is electrostatically charged and sprayed onto the part.
The part is
then placed in an oven and the powder particles melt and coalesce to
form a continuous film.
There are two
main types of powder available to the surface finisher:
Thermoplastic
powders that will remelt when heated, and
Thermosetting
powders that will not remelt upon reheating. During the curing
process (in the oven) a chemical cross-linking reaction is triggered
at the curing temperature and it is this chemical reaction which
gives the powder coating many of its desirable properties.
Preparation
The basis of
any good coating is preparation. The vast majority of powder coating
failures can be traced to a lack of a suitable preparation.
The
preparation treatment is different for different materials.
In general,
for all applications the preparation treatment for aluminium is as follows:
Clean
Or Clean
Rinse
Rinse
Etch
Rinse
Chromate
Phosphate
Rinse
Demin Rinse
Oils and
greases are removed in weak alkali or neutral detergent solutions and
the surface is etched to remove heavy oxides. After rinsing, the
aluminium is dipped into a chromate or phosphate solution to form a
conversion coating on the aluminium. This film is chemically attached
to the aluminium. After rinsing the aluminium is finally rinsed in
demineralised water. Some non-chrome, dried in place pretreatment is
beginning to come onto the market; currently, these are not
recommended for exterior applications.
The conversion
coating has two functions:
It presents a
surface to the powder which favours adhesion more than the oxides
that form very readily on aluminium surfaces, and
It reduces the
incidence of under film corrosion, which may occur at holidays in the coating.
The use of
demineralised water reduces the presence of chemical salts on the
aluminium surface. These salts have been found to cause filiform
corrosion in humid conditions.
For steel the
preparation for interior applications may be:
Clean
Rinse
Derust
Rinse
Iron Phosphate
Rinse
Acidulated Rinse
For exterior applications:
Clean
Rinse
Etch
Rinse
Grain Refine
Zinc Phosphate
Rinse
Acidulated Rinse
The grain
refiner is used after acid cleaning of steel surfaces and before zinc
phosphating, otherwise the zinc phosphate coatings produced will be
very coarse with low adhesion. The powder coating applied to a coarse
phosphate will produce rough coatings (a little like
"sandpaper") and possess low adhesion.
For hot dipped
galvanized coatings, which have been stored for more than about 4
hours before powder coating, the following process is necessary for
exterior applications.
Clean
Rinse
Etch
Rinse
Grain Refiner
Rinse
Zinc Phosphate
Acidulated Rinse
The etch is
required to remove the zinc corrosion products which begin to form
almost immediately the zinc is removed from the galvanizing kettle.
The grain refiner ensures a fine phosphate is produced.
How is it done
-- electrostatic spray?
The powder is
applied with an electrostatic spray gun to a part that is at earth
(or ground) potential.
Before the
powder is sent to the gun it is fluidised:
to separate
the individual grains of powder and so improve the electrostatic
charge that can be applied to the powder and
so that the
powder flows more easily to the gun.
Because the
powder particles are electrostatically charged, the powder wraps
around to the back of the part as it passes by towards the air
offtake system. By collecting the powder, which passes by the job,
and filtering it, the efficiency of the process can be increased to
95% material usage.
The powder
will remain attached to the part as long as some of the electrostatic
charge remains on the powder. To obtain the final solid, tough,
abrasion resistant coating the powder coated items are placed in an
oven and heated to temperatures that range from 160 to 210 degrees C
(depending on the powder).
Under the
influence of heat a thermosetting powder goes through 4 stages to
full cure.
MELT, FLOW,
GEL, CURE
The final
coating is continuous and will vary from high gloss to flat matt
depending on the design of the powder by the supplier.
Powder coating guns
There are at
east three types of electrostatic guns in use:
Corona
charging guns where electric power is used to generate the
electrostatic charge. Corona guns are either internal or external charging.
Tribo charging
guns where the electrostatic charge is generated by friction between
the powder and the gun barrel.
"Bell"
charging guns where the powder is charged by being "flung"
from the perimeter of the "bell"
Not all powder
is applied using guns. One system makes use of electrostatic tunnels.
How is colour introduced?
Colour is
added to powder coatings during the manufacturing process, ie before
the powder reaches the powder coater. There is little that can be
done to change the colour consistently, once the powder leaves the
manufacturing plant.
Why powder coat?
Powder coating
produces a high specification coating which is relatively hard,
abrasion resistant (depending on the specification) and tough. Thin
powder coatings can be bent but this is not recommended for exterior applications.
The choice of
colours and finishes is almost limitless, if you have the time and
money to have the powder produced by the powder manufacturer.
Powder
coatings can be applied over a wide range of thickness. The new
Australian Standard, "AS/NZS 4506 - Thermoset powder
coatings", will recommend 25 micron minimum for mild interior
applications and up to 60 micron minimum for exterior applications.
Care must be exercised when quoting minimum thickness because some
powder will not give "coverage" below 60 or even 80 micron.
"Coverage" is the ability to cover the colour of the metal
with the powder. Some of the white colours require about 75 micron to
give full "coverage". One of the orange colours must be
applied at 80 micron.
Colour
matching is quite acceptable batch to batch.
Installations
and maintenance
During
installations, the powder coating should be protected from damage due
to abrasion and materials of construction such as mortar and brick
cleaning chemicals.
Once
installed, maintaining the initial appearance of a powder coating is
a simple matter. The soot and grime which builds up on surfaces from
time to time contains moisture and salts which will adversely affect
the powder coating and must be removed. Powder coatings should be
washed down regularly (at least once each 6 months in less severe
applications and more often in marine and industrial environments).
The coating should be washed down with soapy water -- use a neutral
detergent -- and rinsed off with clean water.
When powder
coated items are installed without damage to the powder coating and
they are maintained regularly, they should be relatively permanent.
The correctly applied coating, although not metallurgically bonded to
the metal will not crack, chip or peel as with conventional paint films.
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