Technology Interchange - Manufacturing Powder Coatings PART TWO Milling and Sifting
Posted on Friday, September 29, 2017
Producing a powder coating is
a multi-step process. It can be
described as being semi-continuous
because it begins as a batch
process (weighing and pre-mixing)
but evolves into a continuous process
(extrusion and milling). In the last
Technology Interchange article, we
discussed the first steps in manufacturing
a powder coating. This consists
of weighing, premixing and extrusion.
This edition covers what happens
to the flakes generated from the
The next step in manufacturing a powder coating involves
reducing the particle size of the chips or flakes that have been
collected off the cooling belt. The goal here is to pulverize these
flakes into a fine powder that possesses a particle size distribution
usable on a customer’s application equipment. This is
typically accomplished with an air-classified mill.
Air-classified mills operate on a system of continuous negative
pressure created by a large fan associated with a baghouse.
This negative pressure draws the flakes into a chamber that
contains a rapidly rotating plate. This plate has pins or hammers
situated on its perimeter. The pins or hammers impact the
incoming flakes. In addition to these high energy collisions, the
rotation of the plate propels the fractured flake into a corrugated
liner that further reduces the particle size. The particles are
continuously smashed in this pulverizing chamber until they are
small enough to escape past the vanes of a high speed classifying
wheel. The rotation of this wheel also creates a buffer of air
which restricts the exit of powder until it is ground to a certain
fine particle size. The classifier speed will effect the ultimate
particle size distribution (PSD).
Formulation variables influence the grinding process. High
molecular weight resins produce powder formulas that are more
difficult to pulverize and may require more cooling of the mill.
Furthermore, feed rates may have to be reduced to allow for
more time in the grinding chamber to achieve targeted particle
size. Resin softening point or glass transition temperature (Tg)
also affects grinding. Low Tg resins can cause fusion in the mill
and may require extra cooling to keep from melting. High Tg
resins can be more difficult to grind as well. In this case, extra
cooling makes the powder more friable and easier to fracture.
The incorporation of high hardness fillers make the formula
more abrasive and causes erosion of internal parts in the mill.
High concentrations of some organic pigments can make cleanup
of a mill arduous. Organic reds and oranges are notorious for
coating internal surfaces of a mill and make cleaning a chore.
Process factors have a significant influence on the ultimate
particle size distribution (PSD). The feed rate of flakes into the
mill affects PSD. Faster rates create less time in the grinding
chamber and will produce coarser particles. The temperatureof the material, the incoming air and the mill itself will affect
particle size. Higher temperature creates coarser particles and
may run the risk of fusing material in the mill.
The speed of the rotor plate which affects the peripheral
speed of the hammers or pins influences PSD. Higher speed
causes more impacts of material and hence a finer particle
size. The speed of the classifier wheel also affects particle size.
Higher speed makes it more difficult for particles to exit the
grinding chamber and therefore creates finer particles. The
speed of the fan on the baghouse affects particle size as well.
Slow speed causes material to pass through the grinder more
slowly creating finer particles.
Sifting and Packaging
From the grinding chamber, the powder is pulled through
a cyclone and drops into a sifting device. The sifting device
eliminates a very small coarse fraction of the ground powder.
This coarse material can be re-introduced into the grinding process
to optimize the yield.
Sifting can be accomplished with either a rotary sifter or
a flat deck screener. With a rotary sifter the powder is introduced
into a chamber consisting of a rotating vanes affixed to
a horizontal shaft. The vanes propel the powder against a cylindrical
screen. The fine particles pass through the screen and
the coarse particles are carried beyond the screen and collected
as “overs.” These coarse particles can be reintroduced into the
mill to improve yield.
A flat deck screener consists of a horizontally oriented
circular screen that is vibrated mechanically and in some cases,
ultrasonically as well. The fine particles pass through the sieve
screen and the coarse particles are carried centripetally off the
screen and collected for reintroduction into the mill.
The screens used in sifters/
sieves are rated either in opening
size (microns) or mesh
(holes per square inch). Screen
size for typical powder operations
range from 80 to 120
mesh (125 to 180 microns).
Powder coatings are
typically packaged into boxes,
barrels or bulk bags/containers
that are lined with a plastic
bag. The plastic bag is typically
3.0 to 4.0 mils (75-100 microns).
The box must possess
adequate strength to withstand
transportation, storage and
handling. A good quality bag
will keep the powder isolated
from the environment keeping
it dry and free-flowing.
The process of manufacturing a powder coating is a
relatively complex semi-continuous endeavor. As the powder
material enters the final steps of milling, sifting and packaging the
formulation is essentially fixed and cannot be altered. Milling is
a complicated process that is influenced by a number of factors
including the nature of the material, and process conditions such
as temperature, feed rates and the speed of grinding components.
The final steps in manufacturing a powder coating have a
critical influence on the particle size distribution and ultimately
the application performance at the powder applicator.
Kevin Biller is technical editor of Powder Coated Tough magazine
and president of The Powder Coating Research Group. He can be
reached via email at firstname.lastname@example.org.