A "fluid bed"
is an open box with a "plenum" chamber underneath,
separated by a porous membrane. Air is introduced into
the lower chamber, which then passes into the upper box
uniformly. The density of free flowing powder in the
upper box is reduced by the air, such that 24 inch deep powder
in a "dead" bed rises to 28 or 30 inches deep.
In a properly moving fluid bed you can reach your hand all the
way down the bottom of the box. Depending on the powder
in the bed, there may be a substantial amount of dusting
coming out of the box. Powders designed for fluid bed
application generally have larger particle size than the
"thin film" eletrostatic powders, and are ideally
spherical in shape to allow easy free flow properties.
The two main uses of a fluid bed are 1) for coating large
irregular items
such as dish washer baskets or pipe fittings. The part
is pre-heated at a
higher temperature than the melting point of the powder (since
the powder
'quenches' or cools the part right away) then dipped into the
fluid bed and
moved around under the surface. The retained heat in the
part and the time
immersed in the fluidizing powder determines the thickness of
the coating.
When the part is pulled out, there may be immediate melting of
the coating,
or it may have a "sugar coating" on the surface.
Usually the part is then
post-heated, ideally in another oven at a lower temperature,
perhaps at or
just above the melt point of the powder. Too high of a
temperature will
cause the coating to sag or even drip. The post heat is
simply to flow out
the coating, since most powders used with this method are
"thermoplastic"
(needing to melt only for property development, and able to
re-melt if it
sees heat again) as opposed to "thermoset"
(cross-linking and chemically
changed after the cure cycle, and will not re-melt).
Common materials used
for this method are Vinyl (PVC), Nylon, and Polyethylene, and
in some cases thick-film epoxies (which do need a cure).
The other use for a fluid bed 2) is with an electrostatic gun.
The powder
is put into the upper box, compressed air is supplied to the
lower box and
adjusted so the powder just moves without dusting too much.
In this case
the top of the fluidizing box is closed to contain the
dusting, with some
sort of filtered vent. The powder is then drawn up to
the gun when the
trigger is pressed. This is generally a superior method
for preparing the
powder for the spray gun compared to the "vibrating
table, auger feed"
method, where an aerated suction hose is dropped into the open
box of powder and the powder is drawn up to the gun. The
suction is easier in application of high volumes of powder,
since the coater just has to set up the box once until it is
empty. With the fluidizer, the powder is scooped or
poured into the upper box as needed. With conventional
homogenous powders, there is probably no difference in
application, but with some "complex" powders such as
Veins, Metallics, and some of Polychem's unique compound
mixes, the fluid bed technique assures a uniform application.
The suction feed method does not allow for any settling or
striation variations which the box may have developed in
transit or use.
Fluid beds may be purchased from electrostatic gun
manufacturers for an arm and a leg. Most Vinyl powder
coaters ( less common anymore, since there have been health
related questions raised in using Vinyl) used to make their
own fluidizers. The walls of the bed can be made from
sealed wood or coated sheet metal, then flanged to the lower
chamber. Fluid beds range from a few inches on a side to
several feet on a side. I know of one coater
who would dip sixteen foot long light poles into a fluid bed
filled with Nylon.
As I understand it, the clever State of California has
declared that each
fluid bed is an air pollution source (whether filled or not,
whether in use
or not) and several coaters have limited their inventory since
each unit has
a license fee.
David R. Collander
PolyChem Industries, Inc.
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