Theory
Infrared energy is a form of radiation, which falls between
visible light and microwaves in the electromagnetic spectrum. (See
Figure 1) Like other forms of electromagnetic energy, IR travels
in waves and there is a known relationship between the wavelength,
frequency and energy level. That is, the energy (temperature) increases
as the wavelength decreases (See the directional arrows in the figure).
Unlike convection, which first heats air to transmit energy to
the part, IR energy may be absorbed directly by the coating. It
may also be reflected or transmitted to the substrate. (See Figure
2) When the equipment is properly matched with the application
either absorption (to heat the powder) or transmission (to heat
the part) may become the primary method used to achieve cure. Because
the energy is radiant (in the form of radiation), IR cure is known
to have limitations based on line of sight. That is, the energy
only travels in a straight line, to be absorbed by sections of the
part facing the source, much like a flashlight illuminating an area
of the part.
Since the thermal conductivity of metals is excellent, some energy
may be selectively transmitted to the substrate resulting in cure
of hidden areas via a conduction mechanism. This allows cure to
be achieved on the inside of a steel tube, for example. Also, in
some cases IR may provide some degree of convection heating, which
also helps to achieve non line-of-sight cure. IR systems are usually
described as high, medium or low intensity. This refers to the energy
level of the source.
•High energy (short wavelength) IR is characterized by bright
visible light which is also emitted. Most of the energy is transmitted
through the coating to be absorbed by the substrate. This type of
cure is therefore best suited for complex part shapes (non line-of-sight
heating) where it is used to heat the substrate. The fastest heat
up rate is possible with high energy IR.
•Medium energy (medium wavelength) IR is the most widely used
for curing because the energy is absorbed directly by the coating.
This type of oven works best with simple or symmetrical parts and
frequently the parts are rotated for uniform exposure. Also, the
oven configuration must match the shape of the part, e.g. ceiling
and floor mounted emitters assist in illuminating the top and bottom
of cylindrical parts.
•Low energy (long wavelength) IR is not effective for cure.
Much of the energy generated is lost to inefficient convection heating.
The fraction of IR energy which does reach the coating is absorbed
at the surface, resulting in possible “skin formation”
or other defects.
Control
Most IR sources emit energy over a band rather than at
a single wavelength or frequency. The broadness of this band, known
as purity, can have a major effect on the cure since it establishes
the degree of penetration. Some systems allow a limited degree of
tuning for control by voltage adjustment. An optical pyrometer can
be built in to show emitter surface temperature which increases
with increasing voltage. More sophisticated units have feedback
loops through programmable logic controllers.
Another control point is the amount of energy available at the
surface of the coating. This is known as the watt density. If the
watt density is too high the coating will scorch and degrade at
the surface. This parameter may be adjusted by decreasing the number
of emitters or by increasing the distance between the emitter and
the part.
Equipment
Infrared radiation can be generated from both electric
and gas sources. In the case of electric infrared, tungsten filament
generally provides short wavelength (@ 3000-4000F surface temperature)
and nichrome filament provides medium wavelength (@ 1800- 2000F).
The filaments are usually encased in a quartz tube. Long wavelength
generators operate at a surface temperature of 1000-1200F and only
40-50% of the electrical energy is converted to IR, with the remainder
resulting in convection heating. Lamps and filaments form the IR
emitter, which is usually accompanied by some sort of reflector
to focus the energy. The walls of the oven may also serve as a secondary
reflector. All require maintenance. Short wavelength emitters have
a life of approximately 5,000 hours at normal voltage settings,
with lower settings resulting in longer life. Reflectors may be
constructed with precious metals such as gold to achieve optimum
reflection and or purity of the IR emissions. An accumulation of
dust or dirt will interfere with the performance of the system.
Gas IR ovens use a flame to heat a ceramic emitter which in turn
generates the IR radiation. “Flameless systems” which
catalytically oxidize the fuel are also available. Gas IR is available
in medium to long wavelength only and the emission spectrum is usually
broader than electric IR, e.g. lower purity. Gas IR is frequently
paired with a gas convection oven to provide faster heat up rates
than convection alone could achieve.
Powder
Coatings for IR Cure
Powders used in combination IR-Convection systems or low
to medium intensity IR-only systems are the same as powders selected
for convection-only cure. That is, these systems generally offer
comparable cure times, in the 8-20 minute ranges. Fast curing powders,
e.g. 400F for 5 minutes or less, are the best candidates for high
or high to medium intensity IR systems. The actual dwell time in
front of the emitters in this case is typically anywhere from 30-90
seconds.
Because powders generally respond well to fast heat up rates, IR can
present some advantages with respect to appearance. High gloss coatings
for example may be even higher in gloss. Unfortunately, low gloss
systems may respond the same way, making gloss prediction a little
tricky. Trial runs or on-line evaluation of lab samples can overcome
this uncertainty. With rapid cure there is also limited opportunity
for flow, therefore low viscosity in the melt is an important characteristic
to overcome orange peel.
Color of the coating can also play a comparatively small role in
the cure process. Black coatings tend to absorb radiant energy more
readily than other colors while whites reflect. This can mean that
oven adjustments may be required to overcome these tendencies.
Application
Advantages and Disadvantages
Two-dimensional parts such as steel blanks and light switch
plates, as well as symmetrical three-dimensional parts such as oil
filters, pen barrels and tubing represent some of the most common
end uses for IR. In the latter cases the parts may be rotated to
achieve uniform radiant energy distribution. Other applications
include heavy castings which would require long heat up times or
very high temperatures in a convection oven. Heat sensitive substrates
such as plastics and composites also receive potential benefits
from IR cure since the coating alone can be heated and the oven
dwell time can be short enough to prevent substrate damage.
IR-only systems do not require high airflows common in convection
ovens. As a result dust and dirt contamination can be minimized.
The lack of combustion byproducts in electric IR makes this a cleaner
process as well.
Despite the potential advantages, many problems must also be investigated
when considering IR cure. The first is that complex part shapes
can be difficult to cure with IR-only ovens. Radiation travels in
straight lines and does not turn corners without the aid of reflection.
Hidden internal surfaces may not cure without the assistance of
conduction or convection.
In addition to part shape, the composition and condition of the
substrate can also represent a source of variation. Individual metals
absorb IR energy differently. This property is known as emissivity.
Gold, which has a very low emissivity, for example, is frequently
used as a reflector because of its low absorption. While common
substrates such as iron, steel and aluminum have emissivities in
the same general range, polishing can change each of these from
good absorbers to reflectors.
Variation in part mass can also be a problem. Systems designed
to cure powder in 60 seconds on 16-gauge steel may provide less
than adequate cure on doubled mass and weld seams due to a heat
sink effect. This is common on the “chime” area of oil
filters, for example. Heat energy is pulled away from the thin substrate
and insufficient energy remains to cure the coating.
IR ovens can be zoned to provide more energy output for the initial
heat up stage and lower output for the leveling or “hold”
stage. Some systems may also allow selected emitters to be turned
on and off for different part types. These variable controls allow
for the greatest degree of flexibility in part mix. Rapid on-off
controls can help to prevent overbake during line shutdown as well
as offering a quicker look at parts after line adjustments.
The most flexible systems offer a degree of control, which actually
exceeds that of convection ovens. If the oven and the powder are
properly matched to the part, a similar “cure window”
may be available. For example, a 120-second dwell time may not cause
problems for a coating designed for a 60-second cure. This is the
same 100% overbake generally built into convection cured coatings.
The real problem is that the absolute time of 60 seconds offers
little chance to react to problems. This has led to a belief that
IR is a less forgiving cure mechanism when compared to convection.
Summary
IR cure can be the right choice for many applications.
Oven suppliers and coaters who know and understand all aspects of
the equipment are best suited to deal with the complexities of these
systems. Symmetrical parts and simple configurations, with limited
to no change, offer the best chances of success. Proven powders
are available for most common applications with the most important
characteristic being reactivity matched to the cure system. When
it comes to IR there is no substitute for testing, whether the intent
is to install new equipment or to introduce a new powder or part
configuration.
