©Copyright 1999/2003 Mario
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
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
- 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.
The basis of any good coating is preparation. The vast majority
of powder coating failures can be traced to a lack of a suitable
The preparation treatment is different for different materials.
In general, for all applications the preparation treatment
for aluminium is as follows:
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
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
- 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
For exterior applications:
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
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.
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 - DIP?
Graphic Powuip16b.tif powqip17a.tif
The powder is fluidised in a hopper and the part is lowered
into the powder cloud. The part can be at room temperature
when an electrostatic charge is applied to the powder or the
part can be preheated to above the melting point of the powder.
The powder particles melt onto and attach themselves to the
hot surface. Control of coating thickness is very difficult
with this process. Both thermoplastic and thermosetting powders
may be used. When using thermosetting powders it is advisable
to return the parts to the oven to complete the curing reactions.
HOW IS IT DONE – ELECTROSTATIC SPRAY?
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
Two types of electrostatic charging is used
- Corona, and
Corona charging can occur within the gun or outside and in
front of the gun. A charged field is generated by electricity.
The powder passes through this field and becomes charged.
In Tribo charging, the charge is always applied within the
gun through the action of friction. The powder particles are
charged when they rub against the wall of the gun (usually
||By increasing the flow speed of the powder
and constricting the spray gun bore an electrostatic charge
is imparted to the powder particles and no powerful electric
fields and lines of force develop between the spray gun
and the object. This often makes it possible to achieve
better powder penetration into otherwise inaccessible
By being less dependent on gun to object distance and the
geometry of the components, the Tribostatic method often produces
a more uniform coating thickness.
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 210C (depending on the powder). Under
the influence of heat a thermosetting powder goes through
4 stages to full cure.
The final coating is continuous and will vary from high
gloss to flat matt depending on the design of the powder by
POWDER COATING GUNS
There are at least three types of electrostatic guns
- 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, while another uses flame spraying
HOW IS COLOUR INTRODUCED?
Colour is added to powder coatings during the powder
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 Australian Standard AS4506:1998 - Metal finishing - Thermoset
powder coatings recommends 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.
Powder coatings were introduced into Australia in the early
70’s and have already settled into a diversity of applications.
- Highly protective, thick powder coatings are applied
to produce linings on the inside of oil drilling pipes where
severe pressures, high temperatures and corrosive materials
are common. Only a select few coatings are satisfactory
in these conditions.
- Reinforcing steel bars for highway and bridge decks are
powder coated to greatly reduce the formation of corrosion
products, which result in concrete spalling and cracking.
The cost to repair damaged concrete is very high.
- With the advances in powder application using flame spraying
techniques, powder coatings can now also be applied in the
- In automotive applications:
- some primers used on bodywork have converted to powder.
This has given improved coating quality and provided
- where a functional as well as a decorative finish
is desirable powder coatings are being used increasingly
- - wheels,
- - bumper bars,
- - mirror frames,
- - oil filters,
- - battery trays,
- - coil springs and
- - heads.
- Specially formulated, hard and scratch resistant
powder coatings are replacing the energy-intensive porcelain
finishes on washer tops and lids.
- Powder coatings are being used on range housings,
freezer cabinets, drier drums, and microwave oven cavities.
- Outdoor furniture, farm and garden implements and
garden tractors also are being finished with powder
coating materials. - Interior fluorescent light reflectors
are coated using a highly reflective, very thin powder
- Exterior lamp housings on highway and parking deck
fixtures are coated with specially formulated powder
which give ultra violet light protection and added corrosion
- Building products:
- Major applications include aluminium extrusions used
for window and door frames
- Kitchen appliances and fittings, bathroom fittings,
interior furniture all can be powder coated.
- Glass, wood products and plastic parts also can be
In other words, everything from spaghetti drying racks to
football stadium seats can be powder coated. More traditional
applications are bicycle frames, air conditioner housings,
small appliances, and office furniture components. The rough
edges sometimes associated with stamped parts can be protected
with powder coatings and the need to grind edges prior to
finishing can be reduced.
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.