Producing a powder coating
is a multi-step process. It
can be described as semicontinuous
because it begins as
a batch process (weighing and
premixing) but evolves into a
continuous process (extrusion
and milling). The exciting part
of the manufacturing process is
taking the abstract concept of
the art of powder formulating
and making it real and tangible.
All the creativity and technology
that has been infused into a
formula becomes a reality. This
Technology Interchange describes
the first part of manufacturing
powder coatings, namely
weigh-up, premixing wand
extrusion.
Weigh-Up
The first process involves weighing the raw materials in
ratios prescribed in the master formula. The weigh-up coincides
with the amount needed by the customer order as well as the
capacity of the premixing vessel. Typical production batches
require a number of premix charges to fill a sales order. Multiples
of weigh-up charges are therefore necessary. Calculating the
overall quantity to weigh at the onset of a powder coating batch
must take into account the estimated yield of the production
processes. Smaller batches inherently incur lower yields mainly
due to “filling the pipeline” of each process. A certain amount of
material is lost when establishing a steady process state especially
in extrusion and milling.
Relatively small batches (less than 500 lbs.) generate smaller
yields than larger batches. A well run large batch process will
run between 95 to 99 percent efficient. Smaller batches run
between 90 and 95 percent. Experience will dictate capability
of processes and guide in determining how much excess raw
material is needed per batch size.
Scales need to be sensitive enough to provide consistency in
the finished powder coating. In general, large bulk components
require less precision than critical additions such as tinting
pigments and catalysts. Plus or minus 1 percent is a good rule of
thumb for weighing accuracy. Keep in mind that ±1 percent of a
small addition can be quite small.
Premixing
The premixing step homogenizes the raw materials mix so
that a consistent feed can be introduced into the extruder. Moreover,
large resin flakes are fractured, which helps facilitate a mix
that can be fed more easily. A high-intensity mixer can achieve
acceptable blending in a relatively short duration (2 to 5 minutes).
Adequate mixing can also be accomplished with a slower,
less intense process that typically can take 10 to 25 minutes.
Consistency can also be enhanced by the order that raw
materials are added to the premixing vessel. “Sandwiching”
minor components (tints, additives, etc.) between bulk items
(resins, extenders, etc.) helps minimize these minor additives
from being flung inside the premixer
onto the vessel wall and thereby compromising
distribution.
Some mixers employ detachable
bowls that offer versatility in managing
multiple mixes with the same mixing
head. This design also allows for ease
in cleaning. Premixing equipment is
designed for an optimal range of vessel
filling. Charges too small will be poorly
mixed because the mixing blades will
be incompletely covered. Excessively
large charges will be poorly blended
due to lack of free space to transport
the contents of the vessel. Excessively
large charges can also tax the drive
mechanism of the mixer to the point of
tripping circuit breakers and damaging
motors and electrical drives.
Extrusion
With a homogeneous dry blend of raw materials, the extrusion
process can commence. The extrusion step can be described
as the “paint-making” process. This is where the powder
coating formula is compounded. At this point the formula
is locked-in, and very little can be altered (with the exception
of particle size reduction) after
this process.
Two types of mixing are
accomplished in successful extrusion.
The resins, additives
and pigments are intimately
distributed to create a consistent
blend. This uniformity of
mixing affords consistent appearance
and performance of
the finished coating. Inconsistent
mixing typically results in
variable gloss, smoothness and
possibly film performance.
Powder coating raw materials can be
blended in a fixed-bowl, high-intensity
mixer in less than 5 minutes.
The distribution facet of
the extrusion process entails
melting the resinous
components and mixing the
non-melting constituents
throughout the molten mass.
This is achieved by introducing
the homogenized dry
ingredients into an extruder.
The goal is to shear the mixture
against a hard surface until it melts. Mechanical force is put
into the mixture. This work increases the ambient temperature
of the mixture and extruder mechanism (i.e., barrel and
screws). The increased temperature combined with the shear
exerted by the extruder screws melts the resinous components,
distributes the non-melting constituents and de-agglomerates
pigments. This is all accomplished as the material is moving
rapidly through the extruder
Compounding powder coatings
requires the dispersion of pigment agglomerates.
Most pigments are supplied
in their natural agglomerated form. The
powder coating extrusion process endeavors
to disperse these agglomerates.
By dispersing the pigment agglomerates,
a more consistent and intense
color is developed. Incomplete pigment
dispersion results in uncontrolled color
consistency.
The extrusion process basically
consists of a feeding mechanism, a
compounding section and cooling. The
material is fed into a rotating mechanism,
usually a screw device encased by a barrel. The screw is configured with flights or nodes that
exert work on the molten mass as it passes through the extruder.
Extruders designed to manufacture powder coatings employ
a single screw or twin-screw mechanism. Single screw types have
flights along the circumference of the screw. These flights convey
the mixture into pins that radiate from the interior of the barrel.
This creates shearing of the material that in turn melts the mass
and affects the distribution and dispersion of the components.
The molten mass exits the barrel and is then cooled and broken
into flakes.
Twin-screw extruders operate on a very similar principle;
however, they use the shearing action of two co-rotating, intermeshing
screws encased in a smooth barrel. Work is exerted into
the mixture by the kneading blocks on the screws. The coordinated
rotation of the screws pushes the material from screw to
screw as it travels the length of the barrel. The heat generated
during this process allows the resinous components to melt. The
shearing action intimately melt mixes raw materials and disperses
the pigments.
Extruders are temperature controlled. This is usually
achieved either through electrical resistance heaters or heater/
cooling units that circulate fluid media through cavities in the
extruder barrel and screw(s). The medium can be oil or a glycol/
water mixture. Resistance heated units are used in conjunction
with media circulating chillers. The initial temperature settings
of an extruder should coincide with the melt point of the major
resinous components. Temperature is typically set slightly above
the melt point of the resin(s). Upon initiating the extrusion
process, the mixture quickly becomes molten and the work put
into it establishes a rather consistent temperature. The role of the
heater/cooling device becomes more of a cooling process at this
point.
Optimal extrusion is achieved by processing a relatively viscous
mass of material. Low viscosity causes incomplete filling of
the free volume between the screws and the barrel that results in
poor compounding. It is therefore important to avoid setting the
temperature too high. Moreover, excessive heat can sometimes
cause pre-gelation of a thermosetting mixture. This can result in
the generation of “seeds,” or high molecular grains of chemically
reacted material. These seeds invariably cause film defects in the
finished coating.
Extruder screw speed can be adjusted. Most powder manufacturers
run their extruders at full speed to provide the highest
output possible. This makes sense in nearly all cases. Sometimes
a formula may contain difficult to disperse components such as
carbon black pigment. In these instances, it may require a slower
screw speed to achieve adequate dispersion.
Extruders can be cleaned efficiently by feeding a suitable
thermoplastic resin after the batch has been compounded. Rigid
grades of polyvinyl chloride and polypropylene can be used to
purge and clean an extruder. The thermoplastic resin is fed into
the extruder and can be reintroduced in its molten state. This action
scrubs the interior of the barrel and the kneading blocks or
flights of screws. The exit die of the extruder should be removed
and cleaned after the purging process because material can hang
up in dead spaces associated with the die. Specific materials have
been developed for cleaning extruders. These products contain
proprietary blends of thermoplastic resins, fillers and wetting
agents and are claimed to be more efficient than off-the-shelf
grades of PVC or polypropylene.
After extrusion, the formula cannot be easily altered. Modification
can be accomplished by re-extruding material with
raw components; however, this process is costly and should be
avoided.
Cooling and Flaking
Extrudate exits the barrel at a relatively high temperature
typically ranging between 212 to 284°F (100 to 140°C). This
molten material needs to be rapidly cooled to avoid pre-reaction,
or “B-staging.” The cooling process involves introducing the
moving extrudate into a set of chilled rolls, which then convey
a ribbon of material onto another cooling surface such as a continuous
belt or rotating drum. After the extrudate is sufficiently
cooled it is broken into flakes that are suitable for feeding into a
comminution or milling process.
Summary
The process of manufacturing a powder coating is a relatively
complex semi-continuous endeavor. The raw materials are committed
early in the process and formula cannot be easily changed
after extrusion. It is therefore very important to control the
quality of the raw materials and the manufacturing process. This
requires a well-trained production staff, vigilance in adherence
to manufacturing procedures, and a steadfast respect for process
control. Keeping these things in the forefront allows successful
manufacture of high quality powder coatings.
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 kevinbiller@yahoo.com