Before
applying the powder coating material you must first consider the substrate
material that you will be coating. It is very important to have
a clear understanding of the true composition of the surface of the
part. The word substrate means; a part, substance, element etc.
which lies beneath and supports another; foundation.
Before processing any part it is first necessary to distinguish
between the "True Surface" and the "Apparent Surface". The
exact composition of the surface must be known in order to select
the proper pretreatment process as well as the correct cure and
temperature cycles.
The "True Surface" is the actual surface that the coating material
will come into immediate contact with. The ``Apparent Surface"
is what you think you are coating over. For example: Powdercoating
over what may be polished brass may surprise you if the material
has a water based clear coat that cannot be seen. The apparent
surface was polished brass, the actual surface was water based clear
coating.
The following is a brief explanation of some of the most common
materials that are powder coated today.
ALUMINUM: A silvery metallic chemical element. It
is remarkable for lightness, malleability, and resistance to oxidation.
Atomic weight = 26.97, atomic number = 13. Melts @ 659.7 degrees
C, or approx. 1220 degrees F. Pretreatment recommendations
for maximum corrosion resistance is chem-film or chromate conversion
coating. Aluminum heats up quicker than steel in a standard
convection oven. This may allow cure process of the powder
to happen in less time resulting in higher line speeds for high
production applications.
ALUMINUM CASTING: Aluminum in its molten state is poured
into a form and allowed to solidify and form the desired part configuration.
Pretreatment recommendations are the same as above. Special
care in processing may be necessary if the casting material has
trapped contaminants or air pockets. Overcoming these types
of problems may involve special powder formulations and/or special
application techniques.
DIE-CASTING & SAND CASTING: Die casting is generally
done under pressure, whereas, a sand casting is simply poured into
the compressed sand impression mold as is. The pressure injected
die-casting process is less porous than the sand-casting process
and subsequently produces less problems than occur by outgassing
during the post curing cycle of the powder coated finishing process.
ALUMINUM and ZINC METAL SPRAY: The process of metal spraying,
also called spray metalizing, consists of applying a protective
metal coating to the surface of a steel substrate. The application
spray gun is constructed so that aluminum powder or wire is
fed to the gun, where it is heated to a molten state. These
molten droplets of aluminum are propelled via nitrogen gas and compressed
air towards the desired surface and fuse together to form a protective
aluminum coating at a predetermined thickness. Metal spraying utilizing
portable equipment is frequently used for metal build up on warn
parts in field repairs. Only recently has this process been
utilized in the form of corrosion protection as a base material
for applications in extreme corrosive environments. (reference,
Alumaseal-90 Corrosion Protection by Metal Flame Spray)
COLD ROLLED STEEL: The most common of the metals encountered
by the jobshop powdercoater, this product is roll formed to a close
tolerance and a fine surface finish, suitable for stamping, forming,
and moderate drawing operations. This material can be bent
flat upon itself without cracking. Good base for phosphate
conversion coating. Pretreatment recommendations are Clean,
Phosphate, rinse, and seal or deionize rinse.
HOT ROLLED STEEL: A low carbon steel suitable for forming,
punching, welding, and shallow drawing. Surface has normal
mill scale that must be mechanically or chemically removed prior
to the application of any conversion coating or any organic topcoat.
This mill scale adheres weakly to the metal and forms a layer between
the desired finishing material and the steel substrate. Thus,
the total adhesion properties of the finish over mill scale would
depend on the weak adhesion of the mill scale to the base metal.
HOT ROLLED STEEL PICKLE AND OIL: A low carbon material from
which the mill scale has been removed by acid pickling. The
light oil is applied after the acid pickling to prevent corrosion
from forming on the steel. This material has a smooth surface,
suitable for stamping, drawing, and pretreatment prior to coating.
CAST IRON: A hard unmalleable pig iron made by casting:
contains between 6-8 percent impurities, including a high proportion
of carbon, and is very fluid and fusible when molten. Melts at 1200
degrees C, or approx. 2190 degrees F.
BRASS: A yellowish metal that is an alloy of copper and
zinc. Melts at 900 +or- degrees C. Highly polished brass resembles
gold, however, this material will tarnish rapidly when exposed to
the environment. A clear coating applied over this polished
surface will reduce this tarnishing effect to a degree. Problems
are often encountered when powdercoating a clear finish over this
material when the copper to zinc ratio is increased. The copper
has a tendency to turn the bright polished gold look to a semi-dull
reddish appearance when exposed to accelerated cure temperatures
commonly used in the powdercoating process. To overcome this
problem one must reduce the maximum temperature that the part is
heated to. This will result in a longer cure time at a lower
temperature. Proper cure of the finish material must be achieved
or the coating will fail.
COPPER: A Reddish-brown malleable, ductile, metallic element
that is an excellent conductor of electricity and heat. Symbol
= Cu, atomic weight = 63.54, atomic number = 29. Melts at 1083 degrees
C, or approx. 1980 degrees F.
LEAD: A heavy, soft, malleable, bluish-gray metallic chemical
element used for piping and in numerous alloys and compounds. Symbol
Pb, atomic weight = 207.21 atomic number = 82. Melts at 327.4
degrees C. or 621 degrees F. This metal cannot be processed
with conventional powdercoating materials due to the low melt temperature
of the metal. Problems are often encountered when parts such
as light fixtures are coated that are constructed of brass and have
LEAD/TIN solder holding the fixture together. The solder will
soften and sometimes melt at the cure temperature required by most
powdercoating materials. A possible solution here would be
to construct the fixture with silver solder which has a much higher
melt temperature.
ZINC: A bluish-white, metallic chemical element, usually
found in combination, used as a protective coating for iron, as
a constituent in various alloys, as an electrode in electric batteries,
and in the form of salts in medicines. Symbol Zn atomic weight
= 65.38 atomic number = 30. Melts at 419.5 degrees C, or approx.
790 degrees F.
ZINC CASTING: Zinc in a molten state is poured into a form
and allowed to solidify and form the desired part configuration.
The zinc material used in this process is sometimes a poor quality
alloy of zinc and can cause outgassing problems. If the molten
zinc or zinc alloy cools too rapidly while it is injected into the
mold form it may cause partial solidification which in turn may
produce air entrapment that will cause outgassing and/or blistering
when the entrapped air expands during the heated cure cycle of the
coating process.
ZINC PLATING: Many types of zinc plating surfaces are available
in a variety of thicknesses. Some will readily accept the
organic coating and some will not. The zinc material itself
generally does not cause any problems but watch out for the brighteners,
wax seals, and other products that are used to prolong the time
in which oxidation of the zinc finish occurs.
The application of any zinc coating as a base coat prior to the
application of an organic coating provides sacrificial protection
as well as the barrier protection that is given by the organic topcoat.
This type of additional protection is also offered by the application
of aluminum and zinc by metal spray. It is important to communicate
to the zinc plater or metal supplier that you intend to pretreat
and apply an organic coating to the surface.
PAINTLOCK - ELECTROGALVANIZED: Produced by electrolytically
depositing zinc on cold rolled steel sheets, then chemically treated
to assure maximum coating adherence to the surface. The smooth,
light gray, thin, pliable coating of zinc permits forming and fabricating
operations without peeling or flaking. This material is excellent
for the powdercoating application. Just enough zinc is applied
to offer the sacrificial protection that is needed if the integrity
of the coating is broken, yet the thin even coating of high quality
zinc is consistent and free of porosity that is common in heavy
deposits of zinc such as hot dipped galvanizing.
HOT DIPPED GALVANIZING: As mentioned this application is
generally of the lowest quality zinc alloy. The thickness
of the application is very difficult to control. It is common
for the plater to melt wax on the surface of the molten zinc to
reduce the amount of harmful vapor fumes that are given off from
the zinc. This wax substance is often deposited onto the parts
when they exit the molten zinc bath. This wax residue will
not be removed by conventional power washing methods.
STAINLESS STEEL: There exists many types of stainless steel
alloys that vary in chemistry and properties. Most of which
are not effected by the pretreatment chemicals that are commonly
used in pretreatment systems. Therefore, one can only expect
to achieve degreasing on this substrate unless the preparation includes
mechanical treatment such as blasting or shot peening. Mechanical
pretreatment will result in a better surface for the organic material
to bond to physically.
A few metal related terms that you should be familiar with are:
SUBSTRATE: From (substratum) meaning; a part, substance,
element, etc. which lies beneath and supports another; foundation.
ALLOY: To reduce the purity of (a metal) by mixing with
a portion of one less valuable. To mix metals so as to form
an alloy. Example; Zinc and Copper to form brass.
CASTING: That which is cast in a mold; anything that is
formed by pouring a liquid into a hollow form and allowing it to
harden or solidify; especially a metal piece so formed.
