heating has many benefits that make it an attractive
curing method for powder coatings. This article discusses
those benefits and the various factors that affect
infrared (IR) curing efficiency, productivity, and
finish quality. As a result, finishers will understand
the options and opportunities offered by IR heating
and, from a practical standpoint, how to get the most
out of an IR heating system.
IR process ovens have been used for decades to cure
powder coated parts, the technology is viewed by some
as powder coating technology was a few years ago:
a sophisticated technology with limited application
and potential. Yet, like powder coating, the advantages
of IR curing have simply overwhelmed its doubters.
No other heating method offers the economic, environmental,
and quality advantages that IR curing technology does.
Some of its advantages follow:
basic properties of IR heating systems make these
advantages possible: (1) the high rate at which energy
can be transferred by IR heat and (2) the fact that
IR heat can be transferred directly from its source
to the coated part without the use of an intermediate
energy carrier, unlike convection heating, which uses
air as its energy carrier.
improvements IR curing brings to a finishing operation
are the result of the proper application of one or
both of these basic properties. Conversely, if one
or both of these properties cannot be applied in an
operation, it is not likely that IR curing will offer
oven types. Regardless of the fuel or heating element
used, IR curing systems generally are one of two distinct
types: a pure IR system or a combination IR and convection
only. Pure IR oven systems transfer heat by radiation.
No air movement occurs except as a byproduct of the
heat transfer process (natural convection) or the
oven exhaust system. Pure IR ovens generally are suitable
for powder coating cure applications in which the
sole requirement is the transfer of a specified amount
of heat. Curing powder coated flat panels is an example.
general, IR curing systems require higher capital
costs than convection ovens of comparable capacity.
The cost per unit of energy input for IR elements
or burners is higher than that of convection elements
or burners. In addition, to achieve accurate temperature
control, more sophisticated sensors and temperature
controllers are required with IR systems compared
with convection systems. A well-designed IR oven will
be divided into smaller zones of control, increasing
the overall control system's cost. However, operating
economies, quality improvements, and production increases
offset the higher initial costs in most applications.
combination. Simply replacing a convection oven with
an IR unit is usually not the best approach to take
to cure powder coated parts unless the parts are simply
shaped. As many converts to IR heating have discovered,
the best curing system for powder coated parts usually
involves a combination of IR and convection heating
can be attributed to the two steps required in the
powder curing process. First, the coating must be
raised to fusion1 temperature without being
disturbed. Since powder particles that have been applied
electrostatically begin to lose their charge during
heat up, they can be disturbed by vibration or excessive
airflow. Second, the powder must be raised to a specific
cure temperature and held at that temperature to complete
crosslinking. Although fusion temperature is unique
to each powder, it often falls between 200°F and
300°F with crosslinking occurring between 300°F
of the dual steps involved in the powder curing process,
a combination IR and convection oven is often the
most suitable powder curing system. The IR stage rapidly
raises the part temperature without air movement.
The convection stage provides accurate temperature
control on all part surfaces, without danger of over
heating the most salient part surfaces. Combination
systems are generally used in powder coat curing applications
to augment IR heat transfer or to improve temperature
uniformity on complex shapes.
and convection differences. There are several important
differences between IR heating and convection heating.
Finishers who understand these differences are better
equipped to determine which heating method-IR alone,
IR and convection combined, or convection alone is
most suitable for their application.
rates. Infrared heating is only suitable for processes
that can use a high rate of heat transfer. Using the
area of a part being heated as a basis, a standard
convection oven is generally designed to transfer
500 to 2,000 BTU/hour-square foot. By contrast, IR
ovens are generally designed to transfer from 3,000
to 25,000 BTU/hour-square foot.
high heat-transfer rate can substantially reduce the
time required to cure parts. Because powder coatings
only require a rise in temperature to fuse, they can
be cured in one-third to one-half of the time that
it takes in a convection oven on production parts.
Greater reductions can be seen on flat, light-gauge
parts of simple geometry
relation. Another significant difference between IR
and convection ovens is that convection ovens are
generally designed to provide an ambient air temperature
not far above the desired part temperature. Stated
another way, convection ovens are generally designed
to heat parts to the equilibrium temperature the part
would reach if left in the oven forever.
ovens, on the other hand, operate with source temperatures
ranging from 600°F to as high as 4000°F. The
equilibrium temperature a part would reach, if left
in the oven indefinitely, would be considerably above
the actual part temperature desired for the specific
coating cure process. Hence, IR ovens do not heat
a part to equilibrium temperature. Instead, they heat
a part to a transient temperature, which depends on
the part's thermal capacity, its exposure time, and
the net heat transfer between the part, its environment,
and the IR sources in the oven.
exposure time in an IR oven must be accurately controlled
to achieve uniform results. Variations in exposure
time from part to part will produce far greater variations
in final part temperature than the same variations
will produce in a convection oven.
very accurate temperature control is desired, it can
be readily achieved in a convection oven by operating
with a long oven residence time and a supply air temperature
equal to or only slightly above desired part temperature.
More sophisticated control strategies are required
to achieve precise part temperatures in an IR oven.
As a result, the control system on an IR oven is more
complex and expensive.
heat transfer. An IR source has what is called line-of-sight
heat transfer. In other words, it transfers heat only
to surfaces it can see. Hence, a pure IR oven cannot
heat all surfaces of a complex shape uniformly. If
no variation in temperature can be tolerated from
surface to surface on a complex part, an IR oven cannot
be used to heat the part or cure its coating. However,
the more tolerance allowed by the characteristics
of the coating used, the more complex a part shape
can be and still cure properly in an IR oven.
no absolute rule can be formulated, pure IR ovens
have been successfully applied to cure powder coatings
on flanged panels, formed angles such as table legs
or stiffening brackets, and tubular parts. Combination
IR and convection ovens have been used for steel file
drawers and rural mailboxes. Shapes to avoid are deep-drawn
parts with hidden recesses or assemblies with hidden
pockets or corners.
oven designers assume that the exhaust temperature
will be equal to the final part temperature. When
designing an IR oven, this assumption must be discarded.
Because the IR energy is transferred directly to the
part being heated, the ambient air temperature in
the oven has no direct relation to the part temperature.
Actual exhaust air temperature will depend on the
exhaust airflow rate, IR source efficiency, and heat
absorbed by the part being cured. Exhaust air temperature
is determined by a heat-balance calculation to properly
size the exhaust and determine proper energy input
for the entire curing system.
properties and curing mechanisms. An oven designer
has to know exactly what the oven must accomplish
to cure a customer's coating. Frequently, a coater
will specify that the oven must maintain the part
at a certain temperature for a certain amount of time,
such as 5 minutes at 350°F. Without additional
qualification, such a specification is useless. Usually,
the specification is based on the time the part takes
to travel through a production or a laboratory oven,
and the reading is based on the oven's temperature
controller, rather than the actual part temperature.
More precise information is needed to design a cure
best way an engineer can get reliable design data
is to ask the powder coating supplier. The supplier's
chemist can provide accurate information on the coating's
curing mechanism. The chemist can also provide actual
curing cycle curves relating residence time and substrate
temperature as mutually dependent variables under
well-defined oven curing conditions.
next best way to get this information is through laboratory
testing. Then the successful test results can be accurately
scaled up to production size with respect to heat
intensity or heat-transfer rate at the part surface.
of testing. Tests are frequently used to design IR
ovens to avoid the need for or to verify the results
of lengthy calculations. As it is rarely possible
to conduct tests in a full-sized oven system, it is
usually necessary to scale up the results of laboratory
or field tests with a temporary setup.
should determine the maximum IR heat intensity at
the part surface to achieve a proper cure in the least
amount of time. Any lesser intensity would yield a
longer oven than necessary; any higher intensity would
deteriorate the part or overbake the coating. It is
more important to find the maximum IR heat intensity
at the part surface than the distance a part should
be from the IR source. The former can be used to scale
up a full-sized oven, the latter cannot.
example, test results of parts at a distance of 6
inches from an IR source in a laboratory oven cannot
be duplicated in a full-sized oven with a large bank
of the same IR sources. A large bank of IR sources
would have to be farther from the part, or the intensity
of the IR sources would have to be reduced. Source
intensity can be reduced by increasing the space between
the elements or by using lower intensity elements.
of IR ovens usually use test equipment that is calibrated
so the heat intensity is known at various distances
between the part and the IR elements. If an IR source
has not been calibrated, builders use a heat sink
whose temperature is accurately measured after a specific
exposure time, or the time it takes to evaporate a
known weight of water in a shallow cup that is recessed
in a block of insulation.
system efficiency. Efficiency is a term frequently
used and misused in connection with processing systems.
A common definition will usually eliminate much of
the disparity in claimed efficiencies by vendors.
For example, in a typical powder-coat curing application,
energy input is distributed among these factors:
absorbed by the product and carried out
absorbed and carried out by the conveyor and tooling
absorbed by phase changes or chemical reactions
within the oven (fusing of powders)
carried out by airflow through the exhaust system
lost through the oven walls, roof, and openings
utility representative can claim 100 percent efficiency
because all of the electrical energy consumed by the
oven is converted into heat energy, useful and necessary
for the process. The skeptic can claim that the efficiency
is zero percent because none of the energy was really
necessary: All of it is eventually thrown away to
the atmosphere. The contention arises when, for example,
one vendor considers the first three energy distribution
factors just listed, and another vendor considers
the first, second, third, and fifth factors. Without
a common definition, both vendors are right. The point
is, be sure you understand a vendor's definition of
efficiency before accepting or considering an efficiency
any project, tests and calculations are the best ways
to determine which heating system is suitable for
an application. Then the final decision can be made
by calculating annual owning and operating costs.
heating sources. Infrared heating sources for powder
coatings may be electric or gas fired. They consist
of an IR radiating source, or surface, operating between
500°F and 4000°F. The energy input per unit
of radiating area determines the specific temperature.
Some IR sources may require reflectors to redirect
the majority of the IR heat emitted toward the parts
within the oven.
sources use the energy of fuel combustion to heat
metal or refractory surfaces to a temperature at which
they will radiate. Combustion byproducts and convection
to the air in the oven carry off a portion of the
energy. As a result, the radiant efficiencies of gas-fired
IR sources are generally from 30 to 60 percent.
IR sources use the flow of current through a resistance
heating element to raise the element itself or surrounding
material to a temperature at which it will radiate.
If none of the element's heat is lost by conduction
or convection, all of the energy is radiated and the
element has a 100 percent radiant efficiency. However,
a portion of the energy is always lost by convection,
depending on the accessibility of the element to air
and its general construction. Hence, radiant efficiencies
for electric sources generally range from 70 to 90
it is tempting to conclude that electric elements
are better than gas-fired burners for IR ovens, radiant
efficiency alone does not necessarily limit the oven
efficiency in an application. In fact, it is quite
possible that a source with a high radiant efficiency
will yield a lower oven efficiency than another source
with a lower radiant efficiency but with other characteristics
making it more suitable for the application.
important, the overall design of the oven can significantly
affect the curing system's efficiency. Factors such
as proper insulation, airflow control, and accurate
temperature control can improve efficiency markedly.
In addition, the cost per BTU of electric energy is
generally higher than that of gaseous fuels, making
it necessary to examine annual owning and operating
costs, rather than element efficiency alone, to arrive
at an economical selection.
IR burner. These IR sources generally rely on heating
a metal or ceramic to an incandescent temperature
in the range of 700°F to 2000°F. Fuel and
air are mixed at correct ratios for combustion and
burned. If the mixture reaches ignition temperature
in the piping or manifold before reaching the burner,
pre-ignition or flashback will occur. This will destroy
burners. Made in a variety of shapes and sizes, impingement
burners include a gas-burner flame firing on refractory
ceramics of various shapes. The surface of the refractory
ceramics radiates the IR. Impingement burners are
generally not susceptible to pre-ignition or flashback,
resist physical damage well, but have relatively low
radiant efficiencies. They are useful in caring systems
requiring combined convection- IR heating as a result
of part geometry. They resist damage from falling
parts and work well in multipass ovens in which cooling
air cannot be provided to the rear of the burner.
Life expectancies are measured in years, unless the
burner is subjected to overfiring.
matrix burners. These burners have porous or perforated
refractory plates mounted on cast iron or formed steel
plenum chambers. The refractory material may be porous
ceramic, refractory blanket, ported ceramic, stainless
steel, or metallic screens. The fuel-air mixture is
supplied under pressure to the plenum chamber. It
passes through the porous matrix to burn on the surface
facing the load. Combustion occurs evenly on the exposed
surface, heating it to incandescence. As the surface
heats up, the flame recedes into the matrix, which
adds radiant energy to the flame.
burners typically operate at surface temperatures
approaching 1850¡F. Cooling of the plenum chamber
on the rear of the burners must be done to prevent
pre-ignition of the combustible mixture. Airflow as
a result of natural convection usually accomplishes
this. Porous matrix burners have the highest radiant
efficiency of the gas-fired IR sources. Modulating
the input fuel provides about a 3:1 turndown capability
in oven heating intensity
burners. Catalytic burners consist of a porous ceramic
or blanket material impregnated with a catalyst, such
as platinum black, through which a combustible air-gas
mixture is fed. They are similar to the porous matrix
units in construction, appearance, and operation,
but the refractory material is usually glass or ceramic
wool. The combustible air-gas mixture oxidizes within
the matrix at temperatures below those normally required
for combustion. No visible flame is produced. These
burners provide low-to-moderate intensities. They
must include an alternate heat source to preheat the
catalyst before production. Usually, electric heating
elements are the alternate heat source.
tube or panel sources. These sources are internally
fired metal tubes or panels. Radiant tubes have a
burner at one end firing down the tube. They typically
operate at surface temperatures up to 1200°F.
Radiant panel systems surround the parts to be heated
with a metal enclosure. Radiation and hot combustion
byproducts scrubbing the enclosure's surface heat
the enclosures exterior. Infrared emission heats the
parts in the enclosure's interior. The combustion
byproducts can be vented or ducted to the convection
portion of the curing oven.
IR sources. Electric IR sources use heat produced
by current flowing in a resistance wire or ribbon
causing the wire and the IR source to reach an incandescent
tube IR sources. These IR sources contain a coiled
nickel-chrome wire lying unsupported within a to 3/8
to 5/8 inch fused quartz tube. Quartz is used because
it is much more transparent to IR rays than other
materials. As a result, higher heat intensities are
achieved. Porcelain or metal terminal blocks cap the
tube. Because quartz tubes are not sealed or filled
with inert gas, the oxidation temperature in air limits
the operating temperature of the resistance wire.
This also limits how closely they can be mounted in
a curing oven to achieve intense heating. Normal operating
temperatures range from 1300°F to 1800°F for
the coil and about 1200°F for the tube.
impact or vibration can easily damage these units,
they stand up well to thermal shock. They must be
mounted horizontally or the internal coil will sag
and short-circuit. Because the element radiates in
all directions, they are usually mounted in a fixture
that contains a reflector. Life expectancy depends
primarily on how close the element operating temperature
approaches its oxidation temperature.
lamps. Quartz lamps (T-3 lamps) are generally made
as 3/8 inch-diameter tubes of various lengths. They
include a fused quartz tube containing an inert gas
and a coiled tungsten filament held straight and away
from the tube by spacers made of tantalum. Filament
ends are embedded in sealing material at the tube
ends. The tube is pinched to hold each of the tantalum
spacers in place to help prevent element sag. Standard
lamps must be mounted horizontally, or nearly so,
to minimize filament sag and overheating of the sealed
ends. A modified design is available for vertical
normal design voltages, quartz lamp filaments operate
at about 4050°F, while the envelope operates at
about 1100°F. At full voltage they have an average
life of 5,000 hours and can withstand higher oven
ambient temperatures than bulbs or tubes. This allows
their use in more closely packed densities to provide
higher output intensities. Reducing voltage even slightly
increases lamp service life significantly.
lamps are usually mounted on banks of reflectors,
which form the oven's sidewalls. Heat-up and cool
down times are short because the mass of the filament
is very low. The ovens are not always insulated; the
lack of insulation helps keep lamp terminals and wiring
cool. In some very high-intensity applications air-cooled
reflectors, water-cooled reflectors, or both are used.
sheath elements. These elements include resistance
heating wire embedded in an electrically insulating
ceramic material enclosed by a tube of steel or alloy.
Tube diameters generally range from 3/8 to 5/8 inch.
Similar elements are used in broilers of electric
oxidation or scaling temperature of the resistance
wire embedded in the tube limits operating temperatures.
Metal sheath elements are quite rugged; have excellent
resistance to thermal shock, vibration, and impact;
and can be mounted in any position. At full voltage,
the elements attain a sheath surface temperature between
1000°F and 1500°F. Higher radiant efficiency
is achieved when these elements are shielded from
direct airflow. The thermal storage of the element's
filler insulation and sheath yield long heat-up and
cool-down times. This can be a disadvantage. However,
unlike most electric elements, inexpensive percentage
timers can be used.
panels. Radiant panels include resistance heating
wire grids or ribbons sandwiched between a thin plate
of electrical insulation on the radiating side and
thermal insulation on the back or cool side. Low-
temperature panels often use thin ceramic papers and
boards as the radiant surface. High-temperature panels
often use Y4-inch-thick quartz or ceramic plates.
Panels come in various dimensions ranging in widths
from 10 to 30 inches and lengths from 12 to 96 inches.
heat generated is typically 12,000 BTU/hour-square
foot. Normal operating temperatures of the radiating
surface are 500°F to 1400°F. The maximum temperature
the radiant surface can withstand and the oxidation
temperature of the resistance wire limits operating
temperatures. Because the entire surface of the element
serves as a radiator, no reflectors are generally
the entire surface emits IR radiation, relatively
high IR intensities can be achieved at lower source
temperatures compared with lamp or tubular sources.
Like sheath elements, most panels heat up slowly but
provide smooth heat control with contactors or percentage
timers. Panel elements generally cost more per BTU/hour
input than other elements. Life expectancies are quite
long (5,000 to 10,000 hours), unless elements are
overheated or damaged some other way.
curing is ideal for transmitting large amounts of
energy to quickly bring a part up to cure temperature.
A convection oven that circulates air at the desired
part temperature is often more suitable for holding
a uniform and constant part temperature. Thus, the
most suitable heating system often includes an IR
stage, followed by a convective stage.
example, a good oven choice for a job shop that runs
a large variety of parts in many shapes, sizes, and
gauges of material on a monorail conveyor would be
a zone of impingement or matrix burners followed by
a forced-convection section. A tubular electric element
could also be used because it resists damage and accommodates
various coating colors.
good oven choice for a job shop that powder coats
similarly sized flat panels of the same metal thickness
in a range of colors, such as an appliance manufacturer,
would be a full-sized IR oven with quartz tubes or
lamps. Quartz tubes would provide rapid control response
and high energy delivery, minimizing overall oven
length and providing accurate, repeatable temperature
rise. Quartz lamps would be less likely to require
adjustments to accommodate various coating colors.
determine the most suitable oven, end users should
compile a list of criteria unique to their parts and
production requirements. They should also become familiar
with the curing characteristics of the powder coatings
they use. Then they can make the best oven choice
for their application.