A Brief Overview
Powder coating was developed by the Germans during WWII. Most people
have at least heard of it, but may not be familiar with the actual process.
It’s best described as dried, finely ground paint which is first applied
to a surface that is then heated wherein it returns to a liquid state and
then cures over a specific period of time at a given temperature. Goes
on dry, flows, cures, cools and you’re done - although it’s a bit more
involved than that.
Powder was first applied by flocking (dipping preheated parts into
powder) and a later refinement was the fluidized bed in which heated parts
are introduced into a box containing a cloud of powder. Both methods are
still in use although are not suitable for much more than shad darts and
industrial equipment.
In the 1960’s the corona discharge electrostatic gun was developed which
imparts a high voltage, low amperage charge to the powder which is then
attracted to the grounded part being sprayed.
This is what the majority of people use although guns range from the
little dopey things to fully automatic arrays of guns and sensors where
the entire operation runs on conveyor belts with minimal operator input.
The advantages of powder over conventional liquid paints are:
Higher film thickness can be achieved without runs or sags
Powders are formulated for a specific application (indoor or outdoor use,
parts that flex, extreme temperatures, etc.)
Properly applied powder coatings are practically indestructible
Reclaim powder booths reduce powder waste to almost zero by recycling the
oversprayed powder and mixing it with virgin powder and
No toxic substances or emissions are involved.
The disadvantages of powder are:
It retains heat so is not suitable for automotive engine or brake components
Clear coats are currently thicker than with liquid paint so the finish
isn’t as nice
Parts must be able to withstand heating
Relatively short shelf life of the powder itself
Imperfections in the substrate will telegraph through and
High cost of application equipment.
Enough of the little overview. If you really want an education,
the Powder Coating Institute offers the industry bible for ninety (US)
bucks or so and you will see just how involved powder coating actually
is compared to this simplified description.
What follows is not the only way to do it; it’s simply what works best
for this writer.
Let’s Shoot Some Powder!
Here’s your typical piece, in this case a Moto Guzzi motorcycle frame.
This is being done in Ferret Vomit Green and because we’re seeking
both beauty and corrosion protection, it’s being sprayed with TGIC Polyester
powder.
The first step is to degrease the part and then to blast off all of
the chrome or paint and for that I used MaxiBlast plastic media and sand.
Worst part by far.
Once the substrate (underlying material) is exposed and the part is
blown clean with compressed air, the part is ready for the conversion coating.
As an old co-worker would say, "This be the key"!
There are a variety of conversion coatings and application methods available;
I use a hot iron phosphate immersion method. The conversion coating can
be viewed as a primer for the powder as it deposits a layer of iron phosphate
for improved adherence and contains a final degreaser.
The chemicals require daily monitoring and upkeep with my Junior Chemist’s
Titration Kit and electronic PH Tester and eventually the bath requires
replacement as it becomes contaminated. Not the kind of thing you’d care
to fall into but it can be neutralized with liquid caustic soda and safely
disposed of down the drain.
The part is submerged into the heated bath for 5 minutes or so, removed
and allowed to drip dry and is then immersed into an overflow tank containing
fresh water to remove the iron phosphate. This REALLY be the key!
The parts are dried with compressed air and a sample part is rubbed
with a white cloth to see just how clean the batch really is. If any residue
comes off on the cloth, the process is repeated.
I should mention that normally the oven is packed with similarly sized
and colored parts as it isn’t cost effective to just do one part at a time
and you don’t want to mix and match colors in the batch due to the high
air circulation in the oven.
The part is then wheeled into the oven to dry and after it’s been removed
and allowed to cool down to room temperature it goes into the powder booth.
The purpose of the booth is to provide a clean, well-lit area to spray
in, keep the airborne powder levels down to a safe level (in high concentrations
powder is explosive) and to contain the overspray. This particular unit
is a non-reclaim booth so the overspray is simply swept up and discarded.
The powder is screened and loaded into the hopper (a pressurized cylinder
which contains and aerates the powder – referred to as fluidizing so it
emerges from the gun as a cloud), the voltage is set for the area being
shot (flat, recessed, small touch ups, etc.) and the appropriate tip is
put on the gun.
The grounded part is then sprayed until it looks like it’s covered in
colored baking flour. Due to the positive charge imparted to the powder
by the gun, the powder adheres to the part being sprayed. If you’re wearing
short sleeves, the gun will also raise all of the hair on your arms which
is pretty creepy the first time it happens.
The coated part is then returned to Big Bertha and here’s the neat part:
it goes into the oven not looking like much of anything but the heat causes
the powder to revert to a liquid state and when it flows out it looks great!
As this particular powder has an 18 minute cure cycle at 375 degrees,
the part is measured with a non contact infrared gun with a laser beam
and when the part itself hits 375 degrees, the timer is set for 18 minutes.
The timer goes, "Bing"!, it comes out to cool and we have a perfectly
finished, durable - in fact almost indestructable - hard plastic coating.
[Editor: I spent many years
in the motorcycle trade, and used to delight in demonstrating just how
good powder coating is compared to stove enamel by belting a coated part
several times with a hammer, much to the horror of the uninitiated onlooker.
The paint would show a few dents, but would remain adhered to the parent
metal. Try that with baked enamel!]
Care and Feeding of Powder
While powder will shrug off most chemicals, DO NOT allow MEK or Trichloroethane
to come in contact with the powder as it will attack the surface. If you're
not sure, try a small test patch in an area where it won't be visible.
You can wax powder coated pieces if you want but it's not necessary.
Whenever possible, use flat washers beneath both nuts and bolt heads.
Excess powder can be removed with either an oiled tap/die for threads
or with a die grinder affixed with an abrasive wheel or cone.
