There are very specialized operations where Powder
Coatings of less than 30 micrometres or with a TG < 40°
C are used in order to produce smooth thin films. One variation
of the dry powder coating process, the Powder Slurry process,
combines the advantages of Powder
Coatings and liquid coatings by dispersing very fine powders
of 1–5 micrometre particle size into water, which then allows
very smooth, low film thickness coatings to be produced.
Powder Coatings
have a major advantage in that the overspray can be recycled.
However, if multiple colors are being sprayed in a single spray
booth, this may limit the ability to recycle the overspray. Please
contact us directly for a free quote
to anodize your parts today!
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The application of powder is very simple. Filtered, compressed
air, usually at 20-30 psi (137-207 kPa) pushes the powder out
of the gun past the electrode which gives the powder a positive
charge. The part being coated is grounded so the positive powder
particles are attracted to it. When the part is completely covered,
the ground is removed and the part is put into the oven.
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There are two main categories
of Powder Coatings: Thermosets
and thermoplastics. The thermosetting variety incorporates a cross-linker
into the formulation. When the powder is baked, it reacts with
other chemical groups in the powder polymer and increases the
molecular weight and improves the performance properties. The
thermoplastic variety does not undergo any additional reactions
during the baking process, but rather only flows out into the
final coating. The most common polymers used are polyester, polyester-epoxy
(known as hybrid), straight epoxy (Fusion bonded epoxy) and acrylics.
Production:
Another method of preparing the surface prior
to coating is known as abrasive blasting or Sandblasting and shot
blasting. Blast media and blasting abrasives are used to provide
surface texturing and preparation, etching, finishing, and degreasing
for products made of wood, plastic, or glass. The most important
properties to consider are chemical composition and density; particle
shape and size; and impact resistance.
Silicon carbide grit blast media is brittle,
sharp, and suitable for grinding metals and low-tensile strength,
non-metallic materials. Plastic media blast equipment uses plastic
abrasives that are sensitive to substrates such as aluminum, but
still suitable for de-coating and surface finishing. Sand blast
media uses high-purity crystals that have low-metal content. Glass
bead blast media contains glass beads of various sizes.
Cast steel shot or steel grit is used to clean
and prepare the surface before coating. Shot blasting recycles
the media and is environmentally friendly. This method of preparation
is highly efficient on steel parts such as I-beams, angles, pipes,
tubes and large fabricated pieces.
Different powder coating applications can require
alternative methods of preparation such as abrasive blasting prior
to coating. The online consumer market typically offers media
blasting services coupled with their coating services at additional
costs.
Powder Application
Processes
The most common way of applying the powder coating to metal objects
is to spray the powder using an electrostatic gun, or Corona gun.
The gun imparts a positive electric charge on the powder, which
is then sprayed towards the object, which is grounded. The object
is then heated, and the powder melts into a uniform film, and
is then cooled to form a hard coating. It is also common to heat
the metal first and spray the powder onto the hot substrate. Preheating
can help to achieve a more uniform finish but can also create
other problems, such as runs caused by excess powder. See the
article "Fusion Bonded Epoxy Coatings"
Another type of gun is called a Tribo gun, which
charges the powder by (triboelectric) friction. In this case,
the powder picks up a positive charge while rubbing along the
wall of a Teflon tube inside the barrel of the gun. These charged
powder particles then adhere to the grounded substrate. Using
a Tribo gun requires a different formulation of powder than the
more common Corona guns. Tribo guns are not subject to some of
the problems associated with Corona guns, however, such as back
ionization and the Faraday Cage Effect.
Powder can also be applied using specifically
adapted electrostatic discs.
Another method of applying powder coating, called
the Fluidized Bed method, is by heating the substrate and then
dipping it into an aerated, powder-filled bed. The powder sticks
and melts to the hot object. Further heating is usually required
to finish curing the coating. This method is generally used when
the desired thickness of coating is to exceed 300 micrometres.
This is how most dishwasher racks are coated.
Electrostatic Fluidized
Bed Coating: Electrostatic fluidized bed application
uses the same fluidizing technique and the conventional fluidized
bed dip process but with much less powder depth in the bed. An
electrostatic charging media is placed inside the bed so that
the powder material becomes charged as the fluidizing air lifts
it up. Charged particles of powder move upward and form a cloud
of charged powder above the fluid bed. When a grounded part is
passed through the charged cloud the particles will be attracted
to its surface. The parts are not preheated as they are for the
conventional fluidized bed dip process.
Electrostatic magnetic
Brush (EMB) Coating: an innovative coating method
for flat materials that applies powder coating with roller technique,
enabling relative high speeds and a very accurate layer thickness
between 5 and 100 micrometre. The base for this process is conventional
copier technology . Currently in use in some high- tech coating
applications and very promising for commercial powder coating
on flat substrates ( steel, Aluminium, MDF, paper, board) as well
in sheet to sheet and/or roll to roll processes. This process
can potentially be integrated in any existing coating line.
Curing
When a thermoset powder is exposed to elevated
temperature, it begins to melt, flows out, and then chemically
reacts to form a higher molecular weight polymer in a network-like
structure. This cure process, called crosslinking, requires a
certain degree of temperature for a certain length of time in
order to reach full cure and establish the full film properties
for which the material was designed. Normally the powders cure
at 200° C (390° F) in 10 minutes. The curing schedule
could vary according to the manufacturer's specifications. The
application of energy to the product to be cured can be accomplished
by convection cure ovens or infrared cure ovens.
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