In fluidized bed powder coating, heated parts are either dipped
directly into a container of fluidized powder or passed through
an electrically charged cloud of powder, which is created above
a container of fluidized powder. Although it seems like a relatively
simple process, questions abound when it comes to fluidized bed
powder coating, its applications, benefits and costs. Compiling
a number of inquiries that have come in from our readers over the
course of the last several months, Products Finishing turned to
a pair of industry experts — Ross Rector and Lloyd Long of Innotek
Powder Coatings, LLC — for a better understanding of the process.
Q: How do I know if fluidized bed powder
coating is a good fit for my products?
Rector: There are several questions that
need to be asked. First, since fluidized bed coating generally applies
a thicker coating, can the end part withstand the dimensional changes?
Unlike electrostatic coating, fluid bed coating will generally smooth
over any small details in the parts, such as embossed serial numbers,
metal imperfections, etc. This can be extremely beneficial for parts
where Faraday Cage effects are problematic. Welded wire products
are good examples. Electrostatic spray has a hard time getting into
the depths of the wire cross welds, whereas fluidized bed coating,
due to the larger heat mass of the overlapping wires, encapsulates
the entire weld.
Long: Also, part complexity and geometry
should be considered. If you have the right configuration, fluidized
bed coating can be the way to go. Parts that will allow the excess
powder to fall off the part work great. Others—with areas where
trapped air or excess powder that will be difficult to displace—should
be avoided.
Q: What are the key differences between
“fluidized bed” and “electrostatic fluidized bed”?
Rector: With fluid bed coating, the part
has to be preheated, immersed in the coating and then flowed out.
Long: Electrostatically assisted fluidized
bed coating does not dip the part into the powder, it generates
a cloud of charged particles (much like a conventional electrostatic
gun) through which a heated or an unheated part passes. It generally
applies a thin coat vs. the thicker coat from fluidized bed coating.
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A heated part sits in
a fluid bed coater. Photo courtesy of Wright Coating Company,
Kalamazoo, MI.
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Q: Can I use the fluidized bed method
to coat products that do not require such a thick coating?
Rector: Not in most cases. It is very difficult
to achieve a thin coating consistently. Film build generally occurs
rapidly over the first five mils. It’s very difficult to get the
part in and out of the fluidized bed coating tank quickly and still
produce a consistent coating over the entire part.
Long: It is possible, but I suggest revisiting
electrostatic spray coating if you are looking for thin coating
thickness under three mils.
Q: Do I need to pretreat my parts any
differently for fluidized bed powder coating than I would for electrostatic
spray powder coating?
Rector: No. The same cleaners and pretreatments
work for both coating methods. To achieve maximum performance, the
part needs to be cleaned and pretreated just as in conventional
electrostatic coating.
Long: If you are using a conversion coating,
check with your supplier to make sure the preheat temperature you
are using are not detrimental to the conversion chemistry.
Q: Are there any drawbacks associated
with the fluidized bed process?
Long: Color changes can be difficult. Where
practical, one should use a dedicated bed for each color. You will
need to initially charge the bed with enough powder to completely
cover the part. For smaller parts such as dish drainers, 50 lbs
may be sufficient. For larger parts such as park benches, 2,000
lbs is not unheard of. It can be a large initial purchase of powder.
After that, you only need to add make up powder as the coating is
used on the part.
Rector: The drawbacks stem from the differences
between fluid bed and electrostatic coating. Building film by heating
and melting the powder to the part can overcome a lot of problems,
but it can create some problems as well. Smaller details of a part,
such as stamped or raised lettering, will most likely be smoothed
over. Also, if the part has details that are close together, the
coating may bridge from one to the other.
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A part recently taken
out of a fluid bed coater with about 10 mils of functionalized
polyethylene applied. Photo courtesy of Wright
Coating Company, Kalamazoo, MI.
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Q: What is required to heat my parts
for use in a fluidized bed powder coating operation?
Rector: Conventional gas fire convection
ovens are most common. However, electric and gas IR and other methods
work fine as well, depending upon part dimensions. The main goal
is to preheat the part above the melting point of the coating so
the film thickness will build when submerged in the coater.
Long: Induction heaters are used as well,
especially on long, continuous parts.
Q: Does the pre-heating stage make
fluidized bed a more expensive process than electrostatic spray?
Rector: Generally not. The overall thermal
requirements for both coating methods are basically the same. Obviously,
fluidized bed coating uses more heat prior to coating, and electrostatic
spray uses more heat after coating, but these usually net out.
Long: There are some cases where lower
overall heating is required with fluidized bed coating since thermoplastics
(the primary coating used in fluidized bed processes) do not require
a cure cycle, only enough time and temperature to flow out the coating.
Some parts with enough thermal mass will hold their heat long enough
that a post heat cycle is not required. The coating flows out without
it.
Q: What are some other benefits of
fluidized bed powder coating?
Long: There is generally less waste since
it’s a 100% transfer efficient coating method. When compared to
electrostatic spray, capital investment in equipment and ongoing
maintenance is most always lower. It’s basically a low-tech coating
method, and once coating parameters are established, there are not
that many things that can go wrong. It is by far the most efficient
method of applying thick film coatings.
Rector: Productivity increases are very
common when switching to a fluidized bed coating process. The ability
to coat an entire rack of parts by a single dip into the coating
tank can be very efficient.
Q: Can the fluidized bed process be
used with both thermoset and thermoplastic powder coatings?
Rector: Yes. However, thermosets are rarely
used and much more complicated due to the heat buildup in the fluid
bed coater, which can cause the powder to cross-link in the coater.
Thermoplastics are much more forgiving in this manner.
Long: Applying thermosets to a pre-heated
part can result in stringing—where cobweb-like strings form coming
off the hot part. This can happen in both fluid bed and electrostatic
spray coating processes. Further complicating matters, most thermosets
are sold in a smaller particle size and will tend to dust out of
the coater into the plant environment.
Q: What are common fluidized bed process
parameters?
Long: There are no common parameters since
it changes dramatically with part thickness. Two-inch thick bar
stock can be coated with functionalized polyethylene by preheating
to 250°F, dip coated and will most likely flow out without any post
heating. Conversely, thin expanded metal may have to be preheated
to 450°F to achieve the desired coating thickness, and then post
heated at 350°F for four minutes to complete the flow out. We have
never been able to come up with coating parameters that work for
everyone. Ovens are different, and parts cool down at different
rates. Substrates, line speeds and environmental conditions all
vary as well.
Rector: Here are some nominal starting
points—take an average part of fabricated wire such as a refrigerator
rack. Preheat it for six min at 500°F and then (within 10 seconds
of heating) dip it for six seconds. Post heat it for one-and-a-half
min at 350°F. This will normally produce a film build between 10–12
mils. On applications such as bike racks, where 30 mils of coating
are desired, heat the part for six min at 550°F, dip it for 30 seconds
and post heat it for one-and-a-half min at 400°F.
Q: Please address the issue of over
spray and 100% transfer efficiency vs. charging a fluid bed coater.
Rector: One of the upsides to fluidized
bed coating is the fact that virtually 100% transfer efficiency
is obtained. Powder either melts to the part or remains in the fluidized
bed. The downside is in the charging of the tank. A tank large enough
for the part must be charged with the coating which—for large parts—can
be a substantial amount of powder.
Long: To boil it down, fluidized bed coating
is a fairly inexpensive system that will give you virtually 100%
transfer efficiency. You can achieve 100% transfer with electrostatic
spray, but it takes a relatively expensive system to get you there.
In many cases, the cost of the initial fluid bed charge is generally
lower than the cost of the electrostatic spray equipment needed
to achieve 100% efficiency.
About the Authors:
Lloyd Long is technical manager for Innotek Powder Coatings,
LLC in Big Spring, TX. He has been in the powder coating and plastic
process industries for more than 18 years and served in numerous
capacities.
Ross Rector is an account executive with Innotek Powder Coatings,
LLC. He has been in the powder coatings industry for more than
30 years in numerous capacities.
