Powder
coating has long been used to coat metal parts because it is an efficient, no
VOC, single-coat process that provides a high-quality, durable finish. Many types
of parts are commonly powder coated, including lawn and garden, appliance, automotive
and architectural. However, powder coating has several disadvantages that have
kept it from being used for some applications. Two of these disadvantages are
the high temperatures and long dwell times required to melt, flow and cure the
powder. Typical curing schedules are 320-390F for 20-60 min depending on part
size and shape. This has typically limited the application of powder to metal
parts that could withstand these conditions and slowed throughput for massive
metal parts.
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UV
Powders
Ultraviolet (UV) curable coatings, inks and adhesives have been used in a variety
of industrial applications for more than 30 years, from beverage can coatings
to printing inks to overprint varnishes. UV curable powder coatings now make it
possible to powder coat parts such as medium density fiberboard (MDF), plastics
and preassembled and diecast metal parts. Plastics are difficult to powder coat
because they are both heat sensitive and nonconductive. Additionally, galvanized
and electroplated substrates that tend to outgas or are temperature sensitive
can now be powder coated with less concern about the likelihood of coating defects.
UV powder coatings may also be a better option for massive metal parts, such as
engine blocks, iron light pole bases, and I-beams, that act as a heat sink during
processing. With a traditional powder coating, the part surface must remain at
high temperature to ensure adequate cure of the heat-activated powder. But, a
massive metal part absorbs the heat, which results in high energy costs and long
oven dwell and cool down times. Since the UV powder is not heat activated, both
the oven temperature and dwell times can be reduced. This offers finishers new
options and provides the opportunity for traditional custom powder coating firms
to expand their markets.
Unlike traditional powder
coatings, UV powder coatings have a component called a photoinitiator, which is
sensitive to UV light. When the photoinitiators in the powder coating are exposed
to UV light, they form free radicals that initiate the polymerization process.
So, one advantage of a UV powder is that the melt and flow can be totally separated
from the curing, allowing superior flow out and leveling of the coating prior
to crosslinking. Most UV powders melt and flow in IR, convection or IR/convection
combination ovens at 175-250F for 2-10 min. Then they are cured in a UV oven in
a matter of seconds.
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Performance properties
of UV powder coatings frequently exceed those of liquid and traditional powder
coatings. For example, one manufacturer's UV powder for metal can be formulated
in glosses ranging from 60 to 100 units. Staining agents applied for 24 hr resulted
in no effect with coffee, mustard, ketchup or alcohol. On iron phosphated steel,
salt spray performance (ASTM B117) at 1,000 hr resulted in 0.125-inch creep or
less. Solvent resistance was 50 double rubs (MEK) without effect. Another manufacturer's
UV powder has been tested on both yellow chromated (Cr+6) aluminum and electrolytic
chromium coated steel. These tests showed excellent adhesion (ISO 2409), impact
resistance (ASTM D2794) and T-bend (ASTM D4145) results. Corrosion results were
good too, passing copper accelerated salt spray tests (ASTM B368) and the filiform
corrosion test (GSB test). Weathering test results also were satisfactory via
both accelerated QUV-A (ASTM G53) testing and Florida natural weathering standards
testing.
Both textured and smooth
coatings are available in a variety of colors, although not as many colors are
available as with traditional powders. Typical coating thickness ranges from 1.5-4.0
mils depending on the appearance and performance properties required.
Process Considerations
Typically UV curing is a very fast, low temperature process that requires little
floor space and produces high-quality coatings. Thus, with a UV powder we get
the best of both worlds-high material utilization, no VOCs, increased productivity,
reduced floor space, low-temperature curing and a high-quality coating.
UV light is produced by
industrial grade ultraviolet emitting lamps, either electrode arc type or electrodeless
microwave-powered lamps. These UV light sources are manufactured to withstand
industrial plant environments and require little maintenance aside from regular
cooling air filter changes, periodic reflector cleaning and bulb replacement.
Microwave-powered electrodeless lamps have been installed on the UV powder lines
in North America due to their longer life, stable energy and wavelength output
and modular design, which allows positioning of individual lamps for 3D curing.
UV powders currently cost
several times more than traditional powder coatings. However, costs are expected
to drop as production volumes increase. Anyone evaluating whether to use UV powder
should remember to factor in this likelihood. One must take into account the cost-effectiveness
attributable to high material utilization as well. Just comparing the cost per
pound will lead to potentially erroneous conclusions. Another cost factor to consider
is that the floor space will be reduced, so the new plant footprint can be smaller
or capacity increased without adding additional floor space. Line speeds will
be faster, so productivity will be increased. Typically there will also be labor
and energy cost savings. All of this must be considered when evaluating UV powder.
A good cost analysis will compare the cost to coat each part factoring in all
these variables.
A typical UV powder coating
line may have a preheat oven and a melt/flow oven. Occasionally a single oven
can fill these two roles. There are at least two reasons for installing a preheat
zone. First, if positioned at the entrance to the flow-out zone, it minimizes
the likelihood of powder becoming airborne and acting as a contaminant in the
flow-out zone. Second, when located before the application area, it assists in
powder deposition on non-conductive parts. The amount of preheat required depends
on the part size and shape. In some applications it may be possible to use the
heat from formation of the part, say following extrusion, molding or fiberglass
pultrusion, to melt and flow the powder, eliminating the need for both preheat
and melt/flow ovens.
It may be possible to retrofit
an existing powder coating line with a UV curing oven. If the part is hot enough
following formation, then the addition of powder spray equipment and the UV curing
units may be all that is needed. It is important to work closely with a knowledgeable
finishing system supplier and, ideally, a team that includes the UV powder supplier
and UV equipment supplier to help you begin evaluating the possibilities.
H&G
Powder Painting
H&G Powder Painting (London, Ontario, Canada) is a custom powder coater with
three powder coatings lines. The company powder coats metal products for automotive,
hardware and lawn and garden customers. About three years ago, Glen McLean, general
manager at H&G, and Rick Grim, plant manager, met with Fusion UV Systems and
a powder formulator to learn more about UV powder coating. Mr. McLean said he
was interested in UV powders because, "It would enable us to expand our powder
coating capabilities to other substrates, such as wood, preassembled parts and
other heat sensitive substrates. I looked at low-temperature thermal cure powder
coating, but UV powder provides a more durable finish, provides more flexibility
and is faster."
In fact, the two managers
visited the UK to see the first UV powder on MDF line in Wales at Stilexo Ltd.
and an early, if not the first, low-temperature thermal cure powder line in England
at Acre & Associates. Mr. McLean stated, "Things seemed to move slowly
over the three years. But just in the last year everything really seems to be
coming together. The raw material companies offered more to formulators. Now the
formulators are all investing heavily in developing UV powders. Then the equipment
suppliers developed UV powder finishing systems to complete the puzzle."
The system incorporates
one spray booth, one oven and 14 microwave-powered F600 UV curing lamps. Mr. McLean
presented the proposed UV curing line to Ken Giles, owner of H&G. Mr. Giles
liked the design flexibility UV powder brought to wood furniture. He was also
convinced that it was the best coating at the lowest cost and may even replace
some liquid coatings.
H&G purchased a 30,000-ft2
facility and installed the system in August, 2001. The company is marketing its
services initially to the office furniture industry, but it is not limiting itself
to MDF substrates. The line is flexible enough to cure many different sizes and
shapes, and H&G expects to gain customers with plastic and glass as well as
heat-sensitive preassembled parts. The company also thinks UV powder will be a
better option for some metal parts that are currently powder coated, such as massive
metal parts, castings and galvanized and electroplated parts.
It's a good idea to do
a preliminary feasibility study to determine some basic parameters, such as line
speeds, capital costs and operating costs. Once you have approval from management
based on the feasibility study that such a system may make sense for your operations,
then work with a finishing system integrator who can help you decide on retrofit
versus new line options. Lab trials will tell you and/or your customer if the
UV powder can meet your coating requirements and help quantify the operating conditions.
Based on the lab trials, you can then get quotes for capital equipment and begin
the detailed evaluation process.
